Abstract: Grain boundaries often exhibit ordered atomic structures. Increasing amounts of evidence have been provided by transmission electron microscopy and atomistic computer simulations that different stable and metastable grain boundary structures can occur. Meanwhile, theories to treat them thermodynamically as grain boundary phases have been developed. Whereas atomic structures were identified at particular grain boundaries for particular materials, it remains an open question if these structures and their thermodynamic excess properties are material specific or generalizable to, e.g., all fcc metals. In order to elucidate that question, we use atomistic simulations with classical interatomic potentials to investigate a range of high-angle [111] symmetric tilt grain boundaries in Ni, Cu, Pd, Ag, Au, Al, and Pb. We could indeed find two families of grain boundary phases in all of the investigated grain boundaries, which cover most of the standard fcc materials. Where possible, we compared the atomic structures to atomic-resolution electron microscopy images and found that the structures match. This poses the question if the grain boundary phases are simply the result of sphere-packing geometry or if material-specific bonding physics play a role. We tested this using simple model pair potentials and found that medium-ranged interactions are required to reproduce the atomic structures, while the more realistic material models mostly affect the grain boundary (free) energy. In addition to the structural investigation, we also report the thermodynamic excess properties of the grain boundaries, explore how they influence the thermodynamic stability of the grain boundary phases, and detail the commonalities and differences between the materials. read less

Abstract: In this work, molecular dynamics simulations are employed to study the nanotribological process of nickel-based polycrystalline superalloy coating. A series of simulations were carried out using the method of repeated friction to explore the influence of frictional force, friction coefficient, grinding groove morphology, wear scar depth, debris flow direction, subsurface damage degree and evolution of defects during the nano-friction process. In addition, the change mechanism of different grain sizes on wear scar depth, frictional force, friction coefficient, and internal damage in the repeated friction process is also explored. The results show that the frictional force is related to the direction of the dislocation slip, and that the friction coefficient change is related to the number of repeated frictions. Moreover, it is observed that the grinding ball has a shunting effect on the formed wear debris atoms, and the shunt point is located at the maximum horizontal radius. We reveal that the grain boundary structure has a strengthening effect. When the grinding ball rubs to the grain boundary, the nucleation of dislocation defects inside the workpiece is obviously hindered by it. Simultaneously, we also find that the closer the subsurface is to the bottom of the grinding ball, the greater the degree of damage to the workpiece by friction. Furthermore, with the grain size decreases that the material begins to soften, resulting in a decrease of frictional force, friction coefficient, and smaller defects are formed inside the workpiece. The research of this work can better clarify the microscopic mechanism of the polycrystalline friction process. read less

Abstract: We present an atomistic study of heterogeneous nucleation in Ni employing transition path sampling, which reveals a template precursor-mediated mechanism of crystallization. Most notably, we find that the ability of tiny templates to modify the structural features of the liquid and promote the formation of precursor regions with enhanced bond-orientational order is key to determining their nucleation efficiency and the polymorphs that crystallize. Our results reveal an intrinsic link between structural liquid heterogeneity and the nucleating ability of templates, which significantly advances our understanding toward the control of nucleation efficiency and polymorph selection. read less

Abstract: Surface bonding is an essential step in device manufacturing and assembly, providing mechanical support, heat transfer, and electrical integration. Molecular dynamics simulations of surface bonding and debonding failure of copper nanocones are conducted to investigate the underlying adhesive mechanism of nanocones and the effects of separation distance, contact length, temperature, and size of the cones. It is found that van der Waals interactions and surface atom diffusion simultaneously contribute to bonding strength, and different adhesive mechanisms play a main role in different regimes. The results reveal that increasing contact length and decreasing separation distance can simultaneously contribute to increasing bonding strength. Furthermore, our simulations indicate that a higher temperature promotes diffusion across the interface so that subsequent cooling results in better adhesion when compared with cold bonding at the same lower temperature. It also reveals that maximum bonding strength was obtained when the cone angle was around 53°. These findings are useful in designing advanced metallic bonding processes at low temperatures and pressure with tenable performance. read less

Abstract: ABSTRACT An understanding of nucleation, growth and coalescence of voids is required to predict the spall fracture. We employ molecular dynamics simulations to investigate the effect of temperature on the nucleation and growth of voids in single crystal copper under triaxial loading condition. We find that the void density decreases with an increase in temperature. The nucleation rate of the voids diminishes as temperature increases for the range of 300–1250 K. The individual void volume fraction evolves with discrete jumps due to coalescence events. The overall void volume fraction at 1250 K appears earlier than that at 300 K indicating the failure of the material earlier at higher temperatures. The void dimensions evolve with discrete jumps due to coalescence of the voids. The void dimensions change with the applied temperature. The void size distribution at each applied temperature follows a polynomial function. read less

Abstract: We studied the melting dynamics of Gold in the form of thin film. The thin film thickness is 9.756 nm. The systems are heated from room temperature up to slightly above melting point, Tm = 1338 K. The evolution of the atoms in each system is followed using Molecular Dynamics (MD) scheme up to 20 ps. Thermodynamics analysis indicated that thin film is suppressed by the uniform heat while the temperature is increasing. Structural analysis compared with Common Neighbor Analysis (CNA) expressed the condition that melting conduction occurred at the end of simulation. read less

Abstract: We explore the size dependencies of melting dynamics of Gold in the form of thin film. The sizes are 4.896 nm, 7.344 nm and 9.792 nm which is comparable between nanoparticle diameter and thin film thickness. The systems are treated by increasing temperature from 300 K to 1400 K. Molecular Dynamics (MD) scheme is employed to follow the trajectories of the systems up to 20 ps. Structure factor analysis indicated that the melting is suppressed by the increasing size. read less

Abstract: We gratefully acknowledge the support from the Mid-Career Researcher Support Program (Grant No. 2019R1A2C2011312) of the National Research Foundation (NRF) of Korea and from the Meta-Structure Based Seismic Shielding Research Fund (Project No. 1.200043.01) of UNIST. We also acknowledge with gratitude the supercomputing resources of the UNIST supercomputing Center. read less

Abstract: The Poisson’s ratios of graphene-copper nanolayered (GrCuNL) composites under tension and compression are investigated by molecular dynamics and theoretical analysis. The Poisson’s ratio of a GrCuNL composite can be tuned by tailoring its repeat layer spacing without changing the topological structures. The effect of constituent nanocrystalline Cu grain size on the Poisson’s ratio is negligible. There are remarkable in-plane anisotropy and tension-compression asymmetry in the Poisson’s ratio due to the chiral difference in compressive stress in graphene layers. A mechanical model considering the chirality and repeat layer spacing is proposed, which can accurately predict the Poisson’s ratio of a GrCuNL composite. For stable GrCuNL composites, the repeat layer spacing should be larger than 2 nm, and their tunable range of Poisson’s ratio is 0.1–0.35. read less

Abstract: The Pd-H system has attracted extensive attention. Pd can absorb considerable amount of H at room temperature, this ability is reversible, so it is suitable for multiple energy applications. Pd-Ag alloys possess higher H permeability, solubility and narrower miscibility gap with better mechanical properties than pure Pd, but sulfur poisoning remains an issue. Pd-Cu alloys have excellent resistance to sulfur and carbon monoxide poisoning and hydrogen embrittlement, good mechanical properties, and broader temperature working environments over pure Pd, but relatively lower hydrogen permeability and solubility than pure Pd and Pd-Ag alloys. This suggests that alloying Pd with Ag and Cu to create Pd-Ag-Cu ternary alloys can optimize the overall performance and substantially lowers the cost. Thus, in this paper, we provide the first embedded atom method potentials for the quaternary hydrides Pd1-y-zAgyCuzHx. The fully analytical potentials are fitted utilizing the central atom method without performing time-consuming molecular dynamics simulations. read less

Abstract: Coalescence-induced droplet jumping has received more attention recently, because of its potential applications in condensation heat transfer enhancement, anti-icing and self-cleaning, etc. In this paper, the molecular dynamics simulation method is applied to study the coalescence-induced jumping of two nanodroplets with equal size on the surfaces of periodic strip-like wettability patterns. The results show that the strip width, contact angle and relative position of the center of two droplets are all related to the jumping velocity, and the jumping velocity on the mixed-wettability superhydrophobic surfaces can exceed the one on the perfect surface with a 180° contact angle on appropriately designed surfaces. Moreover, the larger both the strip width and the difference of wettability are, the higher the jumping velocity is, and when the width of the hydrophilic strip is fixed, the jumping velocity becomes larger with the increase of the width of the hydrophobic strip, which is contrary to the trend of fixing the width of the hydrophobic strip and altering the other strip width. read less

Abstract: The electronic properties of azobenzene (AB) in interaction with gold clusters and adsorbed on the Au(111) surface are investigated by adopting a near-Hartree-Fock-Kohn-Sham (HFKS) scheme. This scheme relies on a hybrid Perdew-Burke-Ernzerhof functional, in which the exact non-local HF exchange contribution to the energy is taken as 3/4. Ionization energies and electron affinities for gas phase AB are in very good agreement with experimental data and outer valence Green's function) calculations. The presence of C-H⋯Au interactions in AB-Aun complexes illustrates the role played by weak interactions between molecular systems and Au nanoparticles, which is in line with recent works on Au-H bonding. In AB-Aun complexes, the frontier orbitals are mainly localized on the gold platform when n ≥ 10, which indicates the transition from a molecular to a semiconducting regime. In the latter regime, the electronic density reorganization in AB-Aun clusters is characterized by significant polarization effects on the Au platform. The accuracy of the near-HFKS scheme for predicting adsorption energies of AB on Au(111) and the interest of combining exact non-local HF exchange with a non-local representation of the dispersion energy are discussed. Taking into account the significant computational cost of the exact non-local HF exchange contribution, calculations for the adsorption energies and density of states for AB adsorbed on Au(111) were carried out by using a quantum mechanics/molecular mechanics approach. The results strongly support near-HFKS as a promising methodology for predicting the electronic properties of hybrid organic-metal systems. read less

Abstract: Molecular machines, such as nanocars, have shown promising potential for various tasks, including manipulation at the nanoscale. In this paper, we examined the influence of temperature gradients on nanocar and nanotruck motion as well as C60 - as their wheel - on a flat gold surface under various conditions. We also compared the accuracy and computational cost of two different approaches for generating the temperature gradient. The results show that severe vibrations and frequent impacts of gold atoms at high temperatures increase the average distance of C60 from the substrate, reducing its binding energy. Moreover, the temperature field drives C60 to move along the temperature variation; still, the diffusive motion of C60 remained unchanged in the direction perpendicular to the temperature gradient. Increasing the magnitude of the temperature gradient speeds up its motion parallel to the gradient, while raising the average temperature of the substrate increases the diffusion coefficient in all directions. The temperature field influences the nanocar motion in the same manner as C60. However, the nanocars have a substantially shorter motion range compared to C60. The relatively larger, heavier, and more flexible chassis of the nanocar makes it more sluggish than the nanotruck. In general, the motion of large and heavy surface rolling molecules is less affected by the temperature field compared to small and light molecules. The results of the study show that concentrated heat sources can be employed to push surface rolling molecules or break down their large clusters. We can exploit a temperature field as a driving force to push nanocars in a desired direction on prebuilt pathways. read less

Abstract: The present work investigates the dominant mechanisms in the plasticity of nano-sized fcc metallic samples. Molecular dynamics simulations of nanopillar compression show that plasticity always starts with the nucleation of dislocations at the free surface, and the crystal orientation affects the subsequent microstructural evolution. The Schmid factor of leading and trailing partials plays a decisive role in leading to the twinning, or slip deformation. A significant difference is observed in the strength of pillars of the same size with different orientations. The power-law equation exponent is completely dependent on the crystal orientations, and a weak or no size effect is observed in the compression of [100]- and [110]-oriented nanopillars. The observed orientation based behaviour decreases by confining the free surface. read less

Abstract: The strength of nanocrystalline (NC) metal has been found to be sensitive to strain rate. Here, by molecular dynamics simulation, we explore the strain rate effects on apparent Young’s modulus, flow stress and grain growth of NC gold with small size. The simulation results indicate that the apparent Young’s modulus of NC gold decreases with the decrease of strain rate, especially for strain rates above 1 ns−1. The rearrangement of atoms near grain boundaries is a response to the decrease of apparent Young’s modulus. Indeed, the flow stress is also sensitive to the strain rate and decreases following the strain rate’s decrease. This can be found from the change of strain rate sensitivity and activation volume with the strain rate. Temperature has little effect on the activation volume of NC gold with small grain size, but has an obvious effect on that of relatively large grain size (such as 18 nm) under low strain rate (0.01 ns−1). Finally, grain growth in the deformation process is found to be sensitive to strain rate and the critical size for grain growth increases following the decrease of strain rate. read less

Abstract: In this investigation, three-dimensional molecular dynamics simulations have been performed in order to investigate the effects of the workpiece subsurface temperature on various nanocutting process parameters including cutting forces, friction coefficient, as well as the distribution of temperature and equivalent Von Mises stress at the subsurface. The simulation domain consists of a tool with a negative rake angle made of diamond and a workpiece made of copper. The grinding speed was considered equal to 100 m/s, while the depth of cut was set to 2 nm. The obtained results suggest that the subsurface temperature significantly affects all of the aforementioned nanocutting process parameters. More specifically, it has been numerically validated that, for high subsurface temperature values, thermal softening becomes dominant and this results in the reduction of the cutting forces. Finally, the dependency of local properties of the workpiece material, such as thermal conductivity and residual stresses on the subsurface temperature has been captured using numerical simulations for the first time to the authors’ best knowledge. read less

Abstract: In this paper, a concurrent multiscale simulation strategy coupling atomistic and continuum models was proposed to investigate the three-dimensional contact responses of aluminum single crystal under both dry and lubricated conditions. The Hertz contact is performed by using both the multiscale and full molecular dynamics (MD) simulations for validation. From the contact area, kinetic energy and stress continuity aspects, the multiscale model shows good accuracy. It can also save at least five times the computational time compared with the full MD simulations for the same domain size. Furthermore, the results of lubricated contact show that the lubricant molecules could effectively cover the contact surfaces; thereby separating the aluminum surfaces and bearing the support loads. Moreover, the surface topography could be protected by the thin film formed by the lubricant molecules. It has been found that the contact area decreases obviously with increasing the magnitude of load under both dry and lubricated contacts. Besides, a decrease in contact area is also seen when the number of lubricant molecules increases. The present study has confirmed that the dimension of lubricated contacts could be greatly expanded during the simulation using the proposed multiscale method without sacrificing too much computational time and accuracy. read less

Abstract: Molecular dynamics simulations (MDs) are carried out for predicting platinum Proton Exchange Membrane (PEM) fuel cell nanocatalyst growth on a model carbon electrode. The aim is to provide a one-shot simulation of the entire multistep process of deposition in the context of plasma sputtering, from sputtering of the target catalyst/transport to the electrode substrate/deposition on the porous electrode. The plasma processing reactor is reduced to nanoscale dimensions for tractable MDs using scale reduction of the plasma phase and requesting identical collision numbers in experiments and the simulation box. The present simulations reproduce the role of plasma pressure for the plasma phase growth of nanocatalysts (here, platinum). read less

Abstract: Current understanding of the origin of icosahedral clusters or icosahedral short-range ordering in undercooled metallic liquids or glasses is based on Frank’s consideration of an isolated icosahedron whose core has lower potential energy than the shell. Using large scale atomistic simulations and statistical analysis of several bcc (body-centered-cubic) and fcc (face-centered-cubic) metals, here we show that the shells of icosahedrons spontaneously formed inside deeply undercooled metallic liquids or glasses in fact have lower (averaged) potential energy than the cores. The shell potential energy deficiency occurs only to the icosahedral clusters but not to the equilibrium-crystal clusters, and, for icosahedral clusters, this deficiency grows with decreasing temperature. Compared with fcc metals, bcc metals exhibit greater potential energy deficiency on the icosahedral shells and produce significantly more icosahedral clusters upon liquid quenching, which explains the higher tendency of bcc metals to be vitrified observed in ultrafast cooling experiments. Inspecting the potential energy deficiency on the icosahedral shells through computation provides a new avenue to the search for amorphous metals (i.e. metallic glasses) with high glass forming ability and processability. read less

Abstract: Experimental results show that the adsorption of the self assembled monolayers (SAMs) on a gold surface induces surface stress change that cause a deformation of the underlying substrate. However, the exact mechanism of stress development is yet to be elucidated. In the present study, multiscale computational models based on molecular dynamics (MD) simulations are applied to study the mechanism governing surface stress change. Distinct mechanisms for adsorption induced surface deformation, namely inter chain repulsion and thiol-gold interaction driven gold surface reconstruction, are investigated. Two different inter-atomic potentials, embedded atom method (EAM) and surface embedded atom method (SEAM), are used in the MD simulations to study the reconstruction induced surface stresses. Comparison of the predicted surface stress changes, resulting from MD and continuum mechanics based models, with observed experimental response, indicate that a modified SEAM based multiscale model can better capture the surface stress changes observed during alkanethiol SAM formation and gold surface reconstruction is the primary factor behind the surface stress change. Inter chain repulsions of SAM are found to have minimal contribution. Also, both the simulations and experiments show that surface stress change increases with surface coverage density and larger grain size. read less

Abstract: ABSTRACT Molecular dynamics simulations were used to study the atomic mechanisms of deformation of nanocrystalline gold with 2.65–18 nm in grain size to explore the inverse Hall–Petch effect. Based on the mechanical responses, particularly the flow stress and the elastic-to-plastic transition, one can delineate three regimes: mixed (10–18 nm, dislocation activities and grain boundary sliding), inverse Hall-Petch (5–10 nm, grain boundary sliding), and super-soft (below 5 nm). As the grain size decreases, more grain boundaries present in the nanocrystalline solids, which block dislocation activities and facilitate grain boundary sliding. The transition from dislocation activities to grain boundary sliding leads to strengthening-then-softening due to grain size reduction, shown by the flow stress. It was further found that, samples with large grain exhibit pronounced yield, with the stress overshoot decrease as the grain size decreases. Samples with grain sizes smaller than 5 nm exhibit elastic-perfect plastic deformation without any stress overshoot, leading to the super-soft regime. Our simulations show that, during deformation, smaller grains rotate more and grow in size, while larger grains rotate less and shrink in size. read less

Abstract: There are two possible thermal transport mechanisms at liquid-gas interfaces, namely, evaporation/condensation (i.e., heat transfer by liquid-vapor phase change at liquid surfaces) and heat conduction (i.e., heat exchange by collisions between gas molecules and liquid surfaces). Using molecular dynamics (MD) simulations, we study thermal transport across the liquid-vapor interface of a model n-dodecane (C12H26) under various driving force conditions. In each MD simulation, we restrict the thermal energy to be transferred across the liquid-vapor interface by only one mechanism. In spite of the complex intramolecular interactions in n-dodecane molecules, our modeling results indicate that the Schrage relationships, which were shown to give accurate predictions of evaporation and condensation rates of monatomic fluids, are also valid in the prediction of evaporation and condensation rates of n-dodecane. In the case of heat conduction at the liquid-vapor interface of n-dodecane, the interfacial thermal conductance obtained from MD simulations is consistent with the prediction from the kinetic theory of gases. The fundamental understanding of thermal transport mechanisms at liquid-gas interfaces will allow us to formulate appropriate boundary conditions for continuum modeling of heating and evaporation of small fuel droplets. read less

Abstract: ABSTRACT There are two types of pop-in mode that have been widely observed in nanoindentation experiments: the single pop-in, and the successive pop-in modes. Here we employ the molecular dynamics (MD) modelling to simulate nanoindentation for three face-centred cubic (FCC) metals, including Al, Cu and Ni, and two body-centred cubic (BCC) metals, such as Fe and Ta. We aim to examine the deformation mechanisms underlying these pop-in modes. Our simulation results indicate that the dislocation structures formed in single crystals during nanoindentation are mainly composed of half prismatic dislocation loops. These half prismatic dislocation loops in FCC metals are primarily constituted of extended dislocations. Lomer–Cottrell locks that result from the interactions between these extended dislocations can resist the slipping of half dislocation loops. These locks can build up the elastic energy that is needed to activate the nucleation of new half dislocation loops. A repetition of this sequence results in successive pop-in events in Al and other FCC metals. Conversely, the half prismatic dislocation loops that form in BCC metals after first pop-in are prone to slip into the bulk, which sustains plastic indentation process after first pop-in and prevents subsequent pop-ins. We thus conclude that pop-in modes are correlated with lattice structures during nanoindentation, regardless of their crystal orientations. read less

Abstract: In this work, we set out to develop a model of gas-phase nucleation in a mixture of copper and argon atoms, which can be further used for analysing macro-systems. Processes occurring at the atomic level are described using coefficients obtained by statistical analysis of molecular dynamic (MD) data on interactions of metal clusters with metal and argon atoms. The MD simulation results are compared with those obtained using the proposed macroscopic model. It is found that the coefficients obtained by averaging the interaction data suitably represent the integral value of the heat of condensation, although result in the smoothing of the energy distribution functions of the clusters. Analysis of the evolution of the number of clusters has shown that the values of their increase rate were lower than those obtained by MD simulation. The conclusion is made, that in order to improve the precision of the developed gas-phase condensation model, it should be supplemented by cluster coagulation. read less

Abstract: Nanoparticle (NP), as a kind of hard-to-machine component in nanofabrication processes, dramatically affects the machined surface quality in nano-cutting. However, the surface/subsurface generation and the plastic deformation mechanisms of the workpiece still remain elusive. Here, the nano-cutting of a single-crystalline copper workpiece with a single spherical embedded nanoparticle is explored using molecular dynamics (MD) simulations. Four kinds of surface/subsurface cases of nanoparticle configuration are revealed, including being removed from the workpiece surface, moving as a part of the cutting tool, being pressed into the workpiece surface, and not interacting with the cutting tool, corresponding to four kinds of relative depth ranges between the center of the nanoparticle and the cutting tool. Significantly different plastic deformation mechanisms and machined surface qualities of the machined workpiece are also observed, suggesting that the machined surface quality could be improved by adjusting the cutting depth, which results in a change of the relative depth. In addition, the nanoparticle also significantly affects the processing forces in nano-cutting, especially when the cutting tool strongly interacts with the nanoparticle edge. read less

Abstract: The effects of temperature and grain size on mechanical properties of polycrystalline copper-graphene nanolayered (PCuGNL) composites are investigated by analytical mechanical models and molecular dynamics simulations. The yield of PCuGNL composites under tension depends on temperature, copper grain size, and repeat layer spacing. Graphene-copper interfaces play the dominant role in the ultimate tensile strength of PCuGNL composites. The optimal range for strengthening of repeat layer spacing is 2-10 nm, and the failure stress of PCuGNL composites is weakly dependent on temperature. An analytical model is proposed to accurately characterize the mechanical behaviors of PCuGNL composites. read less

Abstract: In order to study the effects of kink-like defects in twin boundaries on deformation mechanisms and interaction between dislocations and defects in twin boundaries under localized load, nanotwinned Cu with two defective twin (TDT) boundaries is compared with the nanotwinned Cu with two perfect twin (TPT) boundaries, and nanotwinned Cu with single defective twin (SDT) boundary and single perfect twin boundary by simulating spherical nanoindentations using molecular mechanics. The indenter force-depth and hardness-contact strain responses were analyzed. Results show that the existence of intrinsic defects in twin boundary could reduce the critical load and critical hardness of nanotwinned material. A quantitative parameter was first proposed to evaluate the degree of surface atom accumulation around the indenter during nanoindentation, and it can be inferred that the surface morphology in TDT changes more frequently than the surface morphologies in TPT and SDT. The atomistic configurations of incipient plastic structures of three different models were also analyzed. We found that the intrinsic defects in twin boundary will affect the incipient plastic structures. The formation of twinning partial slip on the defective twin boundary happens before the contact of the dislocation and twin boundary. The kink-like defects could introduce Frank partial dislocation to the twin boundary during interaction between dislocation and twin boundary, which was not detected on the perfect twin boundary. In addition, the area of twinning partial slips on the upper twin boundary in the incipient plastic structures in SDT and TDT are larger than the twinning partial slip area in TPT, which results in the reduction of the critical hardness in SDT and TDT. The kink-like defects could also block the expansion of twinning partial slip on the twin boundary. Furthermore, we investigated the dislocation transmission processes in three different models. It is found that the dislocation transmission event could be delayed in model containing single defective twin boundary, while the transmission process could be advanced in model containing two consecutive defective twin boundaries. The quantitative analysis of dislocation length was also implemented. Result shows that the main emitted dislocation during nanoindentation is Shockley partial, and the dislocation nucleation in SDT and TDT is earlier than the dislocation nucleation in TPT due to the existence of defects. It is inferred that the intrinsic defects on twin boundaries could enhance the interaction between dislocations and twin boundaries, and could strongly change the structure evolution and promote the dislocation nucleation and emission. These findings about kink-like defects in twin boundaries show that the inherent kink-like defects play a crucial role in the deformation mechanisms and it should be taken into consideration in future investigations. Single defective twin boundary structure is recommended to delay the transmission and block the expansion of twin boundary migration. Some of the results are in good agreement with experiments. read less

Abstract: Significance We demonstrate that an accurate estimation of the displacements associated with the transformation of liquid into crystal is necessary to explain the striking variations in the temperature dependence of the addition rate of liquid atoms to the growing crystal interface during freezing. An assignment algorithm, adapted from operations theory, is shown to provide a good estimate of these atomic displacements. As the assignment algorithm requires only initial and target locations, it is applicable to all forms of structural transformations. In resolving a fundamental feature of the kinetics of freezing, a phenomenon of central importance to material fabrication, this paper also provides the tools to open lines of research into the kinetics of structural transformation. The atomic displacements associated with the freezing of metals and salts are calculated by treating crystal growth as an assignment problem through the use of an optimal transport algorithm. Converting these displacements into timescales based on the dynamics of the bulk liquid, we show that we can predict the activation energy for crystal growth rates, including activation energies significantly smaller than those for atomic diffusion in the liquid. The exception to this success, pure metals that freeze into face-centered cubic crystals with little to no activation energy, are discussed. The atomic displacements generated by the assignment algorithm allows us to quantify the key roles of crystal structure and liquid caging length in determining the temperature dependence of crystal growth kinetics. read less

Abstract: Conventional polycrystalline metals become stronger with decreasing grain size, yet softening starts to take over at nanometer regime, giving rise to the strongest size at which the predominate strengthening mechanism switches to softening. We show that this critical size for the onset of softening shifts from ~12 nm for homogeneous nanograined copper to ~7 nm for its gradient nanograined counterpart, and this strongest size decreases with increasing the grain size gradient. The decrease in the strongest size is prompted by mitigation of grain boundary-mediated softening processes accompanying by enhanced intragranular plastic deformations. We found the nanograins as small as 6 nm, mainly involving intergranular sliding in homogeneous structures, reveal anomalous plastic deformation in gradient systems, which is mediated by partial dislocations nucleation and motion. The results on extended dislocation slip and gradient stress and plasticity, stemming from the structure heterogeneity, shed light on an emerging class of heterogeneous nanostructured materials of improved strength-ductility synergy. read less

Abstract: The theory of phononic friction attributes that the multiphonon processes are the main cause of the mechanical energy dissipation in a wear-free friction process. Unfortunately, it is still impossible to set up a direct relationship between the phonons and the frictional force. In this study, a classical molecular dynamics simulation model is used to mimic a piece of graphene sliding over a supported graphene substrate. It is found that the lifetime of some phonons, especially the modes around the Γ point of the first Brillouin zone, gradually decreases with the increase of the sliding velocity. A phonon lifetime-based model is proposed to explain the variation of the frictional force as a function of the sliding velocity, i.e., the shorter phonon lifetime corresponding to a higher friction force under the same temperature. This model is consistent with the traditional Prandtl-Tomlinson model at a low sliding velocity range, which predicts that the friction force increases logarithmically with the sliding velocity. Once the sliding velocity exceeds a critical value, the lifetime of the excited phonons is far longer than the time for the tip sweeping a lattice constant. In this case, the excited phonons do not have enough time to dissipate the mechanical energy, which leads to the reduced friction force with the increase of the sliding velocity.The theory of phononic friction attributes that the multiphonon processes are the main cause of the mechanical energy dissipation in a wear-free friction process. Unfortunately, it is still impossible to set up a direct relationship between the phonons and the frictional force. In this study, a classical molecular dynamics simulation model is used to mimic a piece of graphene sliding over a supported graphene substrate. It is found that the lifetime of some phonons, especially the modes around the Γ point of the first Brillouin zone, gradually decreases with the increase of the sliding velocity. A phonon lifetime-based model is proposed to explain the variation of the frictional force as a function of the sliding velocity, i.e., the shorter phonon lifetime corresponding to a higher friction force under the same temperature. This model is consistent with the traditional Prandtl-Tomlinson model at a low sliding velocity range, which predicts that the friction force increases logarithmically with the sliding... read less

Abstract: Comprehensive molecular dynamics simulations are conducted to unravel the mechanics and mechanisms associated with the strength and fracture behavior of a highly ordered gold nanowire (Au-NW) array of a pair of nanofasteners (nanoconnectors) under externally applied shear strain. Large-scale atomic/molecular massively parallel simulator (LAMMPS and embedded atom method were adopted to model the atomic interactions of a number of neighboring nanofasteners. This was affected via the use of a periodic simulation box around a pair of highly ordered nanotube arrays to minimize the cost of the computations. Energy minimization using a conjugate gradient algorithm was first performed and followed by atomic relaxation to achieve an equilibrated configuration under the canonical ensemble of constant temperature and volume. The relaxed equilibrated configuration of the nanofastener was then subjected to an externally applied shear strain γ x y at a rate of γ ̇ x y = 0.1 per nanosecond under the canonical ensemble. Our results reveal the importance of the morphology and the overlap depth of the mating nanowire arrays upon the mechanical and fracture behavior of the nanofastener under shear loading. Our work also disclosed the phenomenon of multiple contacts of some displaced nanowires with their neighbors even after their fracture leading to multiple cold-welds with added redundancy to the nanofastener. Finally, in this research, we identified the locations of dislocation emissions and the resulting fracture processes that govern the mechanical integrity and ultimately the functionality of the Au-NW connector. The proposed highly ordered alignment, as conceived numerically herein, can yield a peak stress two to three times higher than that corresponding to a random alignment reported in a previous study. The Au-NW connector also exhibited resistance to fracture, even in cases where small overlap depth is considered in joint bonding. The nanoconnector was also tested at high temperatures (up to 450 K). Our results show that the rising temperature only leads to a minor reduction in the load transmitted by the nanoconnector. read less

Abstract: In this work, we propose a classical equivalent two-body (ETB) model that can capture more detailed dynamic features arising from energy dissipation and atom oscillations, by introducing the Langevin equation of a harmonic oscillator. The trapping probability, scattering angle and the residence time of Ar interacting with Pt (111) and W (110) surfaces predicted by the ETB model agree well with the measured experimental data or molecular dynamics simulations. Moreover, the ETB model is also used to study the influence of oscillating and dissipating properties on the thermal accommodation coefficients and rainbow scattering of gas atoms colliding with the surface. It is found that the dependence of energy accommodation coefficients and rainbow scattering on the oscillating and dissipating parameters shows nonlinear behaviors, and the associated mechanisms are disclosed. ETB model further provides the possibility to explore the physics beyond the existing two-body models for a better description of energy transferring during atom surface interactions. read less

Abstract: Bulk metallic glasses (BMGs) have drawn much attention due to their interesting mechanical properties such as extraordinary elastic strain limits and a high tensile yield stress [1-6]. For example, their yield strengths can be up to 1 to 5GPa and elastic strain limits up to ~2% [7-10]. However, their use for engineering applications has been challenging since BMGs exhibit localized strain softening leading to failure and brittleness. The underlying atomicscale plastic mechanisms are believed to be mediated by a local microscopic mechanism [11-14]. Such localized processes have been observed during high-strain deformation atomistuc simulations [15-22] inspiring the development of the shear transformation zone (STZ) concept and the effective temperature theory of athermal glass plasticity [23-25]. read less

Abstract: Platinum single crystal basal planes consisting of Pt(111), Pt(100), Pt(110) and reference polycrystalline platinum Pt(poly) were subjected to various potentiodynamic and potentiostatic electrochemical treatments in 0.1 M HClO4. Using the scanning flow cell coupled to an inductively coupled plasma mass spectrometer (SFC‐ICP‐MS) the transient dissolution was detected on‐line. Clear trends in dissolution onset potentials and quantities emerged which can be related to the differences in the crystal plane surface structure energies and coordination. Pt(111) is observed to have a higher dissolution onset potential while the generalized trend in dissolution rates and quantities was found to be Pt(110)>P(100)≈Pt(poly)>Pt(111). read less

Abstract: Additive manufacturing (AM) is an emerging and promising technology. In this manuscript, an attempt is made to simulate an AM process via molecular dynamics simulations. Amorphous Cu pillars are built by bundling melting Cu wires parallelly one by one, with a particular temperature-controlling procedure through the use of ‘mediate temperature’. Thus, the volume fraction of the amorphous phase becomes adjustable. Uniaxial tests are conducted on the pillars after free relaxation. The mediate temperature is found to play a profound role in atomic arrangement, which governs the volume fractions of amorphous and crystalline phases. The results obtained also show that crystallization prevails when the pillar is subjected to an external tension. Furthermore, such a stress-induced crystallization serves as the dominant plastic mechanism instead of dislocation, and a vibrating uniaxial loading is found to accelerate the transformation from an amorphous to a crystalline phase, compared with a monotonic tension. read less

Abstract: The idea that shape and structure determines functionality is one of the leiv-motifs that drives research and applications on fields such as catalysis and plasmonics. The growth and stability of metallic clusters is extensively discussed through faceting and energy minimization mechanisms, respectively. Facet truncations on the regular Mackay-icosahedron (m-Ih) give rise to two sub-families exhibiting five-fold symmetry and external decahedral shape. Such successive truncations made to the regular m-Ih, led to a decahedral motif called ‘Decmon’ (Montejano’s decahedron). This structure expose facets (111) and (100), that after a total energy minimization through molecular dynamics simulations using the embedded atom model, proved to be thermally stable. This result has been confirmed by using nano-thermodynamics. The surface energy competition between the (111) and (100) facets explains its stability at some given cluster sizes, and this truncation path permits to glimpse the potential energy surface in the growth path of nanoparticles from the decahedral (s-Dh) to icosahedral (m-Ih) structures. read less

Abstract: The mechanical properties of graphene-Cu nanolayered (GCuNL) composites under bend loading are investigated via an energy-based analytical model and molecular dynamics (MD) simulations. For an anisotropic material, if it has a weak strength in a certain direction, improving the mechanical properties along this direction is normally difficult for its composites. Here, we find that the flexibility of GCuNL composites can be improved considerably by graphene interfaces, despite graphene's small bending stiffness. The graphene interfaces can delocalize slip bands in the inner Cu layers of GCuNL composites, and impede local nucleation of dislocations, thus greatly increasing the yield and failure bend angles. As the thickness decreases, the flexibility of GCuNL nanofilms increases. However, the GCuNL nanofilms are thermodynamically unstable due to interface instability when the repeat layer spacing is less than 2 nm. The energy-based analytical model for large deformation can accurately characterize the bending response of GCuNL nanofilms. read less

Abstract: We implement non-equilibrium Green's function (NEGF) calculations to investigate thermal transport across graphene/metal interfaces with interlayer van der Waals interactions to understand the factors influencing thermal conductance across the interface. It is found that interfaces with a smaller interfacial lattice mismatch, lighter metal substrate and stronger interfacial bonding strength will show better interfacial thermal transport abilities. Strain induced by the interfacial lattice mismatch in graphene is the key factor for the decrease of interfacial phonon transmission in the main frequency range of metals, which finally results in a decrease of interfacial thermal conductance. A comprehensive interfacial influencing factor is proposed combining the factors of graphene density, metal density and interfacial binding energy to realize the prediction of interfacial thermal conductance across the graphene/metal interface. The results are hoped to promote the understanding of the thermal transport mechanism and design of graphene based 2D/3D materials interfaces. read less

Abstract: Electrodes embedded in nanopores have the potential to detect the identity of biomolecules, such as DNA. This identification is typically being done through electronic current measurements across the electrodes in a solvent. In this work, using quantum-mechanical calculations, we qualitatively present the influence of this solvent on the current signals. For this, we model electrodes functionalized with a small diamond-like molecule known as diamondoid and place a DNA nucleotide within the electrode gap. The influence of an aqueous solvent is taken explicitly into account through Quantum-Mechanics/Molecular Mechanics (QM/MM) simulations. From these, we could clearly reveal that at the (111) surface of the Au electrode, water molecules form an adlayer-like structure through hydrogen bond networks. From the temporal evolution of the hydrogen bond between a nucleotide and the functionalizing diamondoid, we could extract information on the conductance across the device. In order to evaluate the influence of the solvent, we compare these results with ground-state electronic structure calculations in combination with the non-equilibrium Green's function (NEGF) approach. These allow access to the electronic transport across the electrodes and show a difference in the detection signals with and without the aqueous solution. We analyze the results with respect to the density of states in the device. In the end, we demonstrate that the presence of water does not hinder the detection of a mutation over a healthy DNA nucleotide. We discuss these results in view of sequencing DNA with nanopores. read less

Abstract: Nitrate (NO3–) is a ubiquitous contaminant in groundwater that causes serious public health issues around the world. Though various strategies are able to reduce NO3– to nitrite (NO2–), a rational catalyst design strategy for NO2– removal has not been found, in part because of the complicated reaction network of nitrate chemistry. In this study, we show, through catalytic modeling with density functional theory (DFT) calculations, that the performance of mono- and bimetallic surfaces for nitrite reduction can be rapidly screened using N, N2, and NH3 binding energies as reactivity descriptors. With a number of active surface atomic ensembles identified for nitrite reduction, we have designed a series of “metal-on-metal” bimetallics with optimized surface reactivity and a maximum number of active sites. Choosing Pd-on-Au nanoparticles (NPs) as candidate catalysts, both theory and experiment find that a thin monolayer of Pd-on-Au NPs (size: ∼4 nm) leads to high nitrite reduction performance, outperforming pu... read less

Abstract: Ductile and brittle materials differ in their physical and mechanical properties and pose distinct interaction with the cutting tool while nano-machining. It is thus imperative to analyse the mechanism of material removal and tool-workpiece interaction. Towards this, molecular dynamics simulation (MDS) is carried out to study the diamond tool and workpiece interaction in the nanoscale cutting of Cu (ductile material) and Si (brittle material). Results show that material removal in Cu takes place through shear deformation by dislocations formation and their propagation while in case of Si, it takes place through phase transformation of the material in cutting zone. Force analysis of both the materials shows that machinability of Cu in nanoscale cutting is better compared to Si. Furthermore, tool wear while machining of Si with sharp edge tool is due to chipping whereas radial distribution function reveals that graphitisation of the round edge tool occurs during machining of Si. read less

Abstract: Surface segregation in bimetallic nanoparticles (NPs) is critically important for their catalytic activity because the activity is largely determined by the surface composition. Little, however, is known about the atomic scale mechanisms and kinetics of surface segregation. One reason is that it is hard to resolve atomic rearrangements experimentally. It is also difficult to model surface segregation at the atomic scale because the atomic rearrangements can take place on time scales of seconds or minutes - much longer than can be modeled with molecular dynamics. Here we use the adaptive kinetic Monte Carlo (AKMC) method to model the segregation dynamics in PdAu NPs over experimentally relevant time scales, and reveal the origin of kinetic stability of the core@shell and random alloy NPs at the atomic level. Our focus on PdAu NPs is motivated by experimental work showing that both core@shell and random alloy PdAu NPs with diameters of less than 2 nm are stable, indicating that one of these structures must be metastable and kinetically trapped. Our simulations show that both the Au@Pd and the PdAu random alloy NPs are metastable and kinetically trapped below 400 K over time scales of hours. These AKMC simulations provide insight into the energy landscape of the two NP structures, and the diffusion mechanisms that lead to segregation. In the core-shell NP, surface segregation occurs primarily on the (100) facet through both a vacancy-mediated and a concerted mechanism. The system becomes kinetically trapped when all corner sites in the core of the NP are occupied by Pd atoms. Higher energy barriers are required for further segregation, so that the metastable NP has a partially alloyed shell. In contrast, surface segregation in the random alloy PdAu NP is suppressed because the random alloy NP has reduced strain as compared to the Au@Pd NP, and the segregation mechanisms in the alloy require more elastic energy for exchange of Pd and Au and between the surface and subsurface. read less

Abstract: Molecular dynamics simulations of structural rearrangements in nanocrystalline Ni with the symmetric tilt grain boundary (GB) ∑5 (310) [001] under shear loading were conducted. It was found that GB can be displaced in the direction perpendicular to the shear loading direction. To activate the displacement, it is necessary to reach the threshold value of the shear stress. The GB displacement is abrupt and is due to a certain sequence of displacements of the atomic planes adjacent to the GB. These planes are successively rebuilt from the structure of one grain to the structure of another grain in the process of GB migration. The velocity of GB migration can reach several hundred meters per second and depends on the rate of shear loading. The use of periodic boundary conditions prevents the rotations of the grains. As the simulated tilt GB is symmetric, both of the crystallite grains will have the same shear moduli in the direction of the applied loading. The shear loading of the crystallite with such a structure does not lead to any volume driving forces. The GB displacement was entirely due to the coupling effect. The shear stress curve as a function of time has a sawtooth shape. The GB experiences displacement upon reaching the maximum value of the applied shear stresses. Despite the high stress values, the GB displacement did not cause the nucleation of the defect structure in the crystallite. The GB migration is accompanied by a change in the volume of atoms involved in structural rearrangements. read less

Abstract: When investigated as a function of deformation rate and time, model nano‐crystals exhibit a clear threshold in mechanical and relaxation behavior, this observation being valid in both the Hooke (elastic) and plastic regime. At low deformation rate (ϵ˙ read less

Abstract: The atomistic stress state at a metal grain boundary is an intrinsic attribute which affects many physical and mechanical properties of the metal. While the virial stress is an accepted measure of the atomistic stress in molecular dynamics simulations, an equivalent definition is not well-established for quantum-mechanical density functional theory (DFT) calculations. Here, we introduce a numerical technique, termed the sequential atom removal (SAR) approach, to reconstruct the atomic stresses near a symmetrical-tilt Σ5(310)[001] Cu grain boundary. In the SAR approach, individual atoms near the boundary are sequentially removed to compute the pair (reaction) force between atoms, while correcting for changes to the local electron density caused by atom removal. We show that this SAR approach accurately reproduces the spatially-varying virial stresses at a grain boundary governed by an embedded atom method potential. The SAR approach is subsequently used to extract the atomistic stresses of the grain boundary from DFT calculations, from which we reconstruct a continuum-equivalent grain boundary traction distribution as a quantitative descriptor of the grain boundary atomic structure. read less

Abstract: In this work, we investigate pressure‐driven water flows in graphene‐coated copper nanochannels through molecular dynamics simulations. It is found that the flow rate in bare copper nanochannel can be significantly enhanced by a factor of 45 when the nanochannel is coated with monolayer graphene. The enhancement factor for the flow rate reaches about 90 when the nanochannel is modified with 3 or more graphene layers. The dipole relaxation time and the hydrogen bond lifetime of interfacial water molecules show that the graphene coating promotes the mobility of water molecules at the interface. The distribution of the potential of mean force and the free energy barriers also confirm that graphene coating reduces the flow resistance and 3 layers of graphene can fully screen the surface effects. The results in this work provide important information for the design of graphene‐based nanofluidic systems for flow enhancement. read less

Abstract: For the first time, this work presents a novel room temperature time-effective concept to manipulate the crystallization kinetics and magnetic responses of thin films grown on amorphous substrates. Conventionally, metal-induced crystallization is adopted to minimize the crystallization temperature of the upper-layer thin film. However, due to the limited surface area of the continuous metal under-layer, the degree of crystallization is insufficient and post-annealing is required. To expose a large surface area of the metal under-layer, we propose a simple and novel approach of using an Au nanodots array instead of a continuous metallic under-layer to obtain crystallization of upper-layer thin films. Spinel cobalt ferrite (CFO) thin film as a 'model' was deposited on an Au nano-dots array to realize this methodology. Our findings revealed that the addition of quantum-sized Au nano-dots as a metal under-layer dramatically enhanced the crystallization of the cobalt ferrite upper layer at room temperature. The appearance of major X-ray diffraction peaks with high intensity and well-defined crystallized lattice planes observed via transmission electron microscopy confirmed the crystallization of the CFO thin film deposited at room temperature on 4 nm-sized Au nano-dots. This crystallized CFO thin film exhibits 18-fold higher coercivity (Hc = 4150 Oe) and 4-fold higher saturation magnetization (Ms = 262 emu cm-3) compared to CFO deposited without the Au under-layer. The development of this novel concept of room-temperature crystallization without the aid of additives and solvents represents a crucial breakthrough that is highly significant for exploring the green and energy-efficient synthesis of a variety of oxide and metal thin films. read less

Abstract: Nanowires (NWs) are one of the fundamental building blocks for nanoscale devices, and have been frequently utilized as mechanical resonators. Earlier studies show that ultra-sensitive vectorial sensing toolkits can be fabricated by changing the flexural mode of NWs to oscillation doublets along two orthogonal directions. Based on in silico studies and the Timoshenko beam theory, this work finds that the dual orthogonal flexural mode of NWs can be effectively controlled through the proper selection of their growth direction. It is found that metallic NWs with a directional-independent shear modulus possess a single flexural mode. However, NWs with a directional-dependent shear modulus naturally exhibit flexural mode doublets, which do not disappear even with increasing slenderness ratio. Further studies show that such a feature generally exists in other NWs, such as Si NWs. Mimicking a pendulum configuration as used in NW-based scanning force microscopy, the cantilevered 110 Si NW demonstrates zeptogram mass resolution and a force sensitivity down to the order of 10-24 N Hz-1/2 (yN Hz-1/2) in both transverse directions. The findings in this work open up a new and facile avenue to fabricate 2D vectorial force sensors, which could enable ultra-sensitive and novel detection devices/systems for 2D effects, such as the anisotropy strength of atomic bonds. read less

Abstract: Many studies have focused on the effect of surface wettability on condensation at the nanoscale, while few studies investigated the condensation process of water vapor below 450K. However, water vapor condensation below 450K is common and important in industrial fields. In this paper, molecular dynamics method is used to study the effect of surface wettability on the performance of water vapor condensation below 450K on a copper surface, and a comparison with the performance of water vapor condensation at 450K was performed. The results show that the heat transfer performance of vapor is not the same when condensing on a hydrophilic surface and on a hydrophobic surface. It’s found that lower temperature vapor requires more time in starting to condense on a hydrophobic surface, whose heat transfer efficiency first increases gradually and finally becomes constant. For the first time the process of vapor condensation on a hydrophobic surface was divided into three stages based on the changes in heat transfer efficiency, and the heat transfer performance of each stage was analyzed. The results show that a stronger surface wettability and higher vapor temperature improve the heat transfer performance during the condensation process. Moreover, the lower the vapor temperature is, the greater the impact of the wettability is on the heat transfer efficiency, and the vapor less easily condenses on a hydrophobic surface.Many studies have focused on the effect of surface wettability on condensation at the nanoscale, while few studies investigated the condensation process of water vapor below 450K. However, water vapor condensation below 450K is common and important in industrial fields. In this paper, molecular dynamics method is used to study the effect of surface wettability on the performance of water vapor condensation below 450K on a copper surface, and a comparison with the performance of water vapor condensation at 450K was performed. The results show that the heat transfer performance of vapor is not the same when condensing on a hydrophilic surface and on a hydrophobic surface. It’s found that lower temperature vapor requires more time in starting to condense on a hydrophobic surface, whose heat transfer efficiency first increases gradually and finally becomes constant. For the first time the process of vapor condensation on a hydrophobic surface was divided into three stages based on the changes in heat transfer... read less

Abstract: The mechanical behavior of nanocomposites consisting of highly ordered nanoporous nickel (HONN) and its carbon nanotube (CNT)-reinforced composites (CNHONNs) subjected to a high temperature of 900 K is investigated via molecular dynamics (MD) simulations. The study indicates that, out-of-plane mechanical properties of the HONNs are generally superior to its in-plane mechanical properties. Whereas the CNT shows a significant strengthening effect on the out-of-plane mechanical properties of the CNHONN composites. Compared to pure HONNs, through the addition of CNTs from 1.28 wt‰ to 5.22 wt‰, the weight of the composite can be reduced by 5.83‰ to 2.33% while the tensile modulus, tensile strength, compressive modulus and compressive strength can be increased by 2.2% to 8.8%, 1% to 5.1%, 3.6% to 10.2% and 4.9% to 10.7%, respectively. The energy absorption capacity can also be improved due to the existence of CNTs. Furthermore, the MD simulations provide further insights into the deformation mechanism at the atomic scale, including fracture in tension, pore collapse in compression and local changes in lattice structures due to stacking faults. read less

Abstract: The nanometric machining of Cu/Ag bilayers and pure Cu film is performed using molecular dynamics (MD) simulations. The mechanical and tribological properties of Cu/Ag bilayers are investigated by comparing with those of pure Cu film. The effects of machining parameters (indenter radius, tool speed and machining depth) on the subsurface damage and material removal are studied by analyzing the dislocation movement, chipping volume, machining force and average temperature of the workpiece. The results show that the hardness of Cu/Ag bilayers is smaller than that of pure Cu film, due to the dislocation nucleation and emission from the Cu/Ag interface. Meanwhile, the friction coefficient of Cu/Ag bilayers is larger than that of pure Cu film. Furthermore, the metal bonding energy at the Cu/Ag interface is weaker than that in pure Cu film, which causes the low hardness in the Cu/Ag bilayers. The Young's moduli in the Cu/Ag bilayers and pure Cu film are calculated by the Hertz contact mechanism and are close to the experimental result. During nanometric machining of Cu/Ag bilayers, the larger indenter radius or higher tool speed would cause a larger indentation force. The chipping volume, machining force and average temperature would increase with the increment of indenter radius, tool speed and machining depth. The subsurface damage can be reduced by selecting the smaller indenter radius, lower tool speed, and smaller machining depth, where fewer lattice defects are produced. In addition, the selection of lower tool speed also plays a crucial role in improving the smoothness of the ground surface. read less

Abstract: In this study, molecular dynamics simulations were carried out to study the coupling effect of electric field strength and surface wettability on the condensation process of water vapor. Our results show that an electric field can rotate water molecules upward and restrict condensation. Formed clusters are stretched to become columns above the threshold strength of the field, causing the condensation rate to drop quickly. The enhancement of surface attraction force boosts the rearrangement of water molecules adjacent to the surface and exaggerates the threshold value for shape transformation. In addition, the contact area between clusters and the surface increases with increasing amounts of surface attraction force, which raises the condensation efficiency. Thus, the condensation rate of water vapor on a surface under an electric field is determined by competition between intermolecular forces from the electric field and the surface. read less

Abstract: The growth and shape stability of bi-metallic cubic Cu-Ni nanoparticles are studied using atomic-level simulations. Cubic nano-crystals coated with an ultra-thin Cu layer can be readily obtained when Ni cubic nanoparticles are used as the seeds. At elevated temperatures, the Cu seed with extending Ni branches preserves its shape compared to the Ni seed with extending Cu branches. read less

Abstract: Conductive modes of atomic force microscopy are widely used to characterize the electronic properties of materials, and in such measurements, contact size is typically determined from current flow. Conversely, in nanodevice applications, the current flow is predicted from the estimated contact size. In both cases, it is very common to relate the contact size and current flow using well-established ballistic electron transport theory. Here we performed 19 electromechanical tests of platinum nanocontacts with in situ transmission electron microscopy to measure contact size and conductance. We also used molecular dynamics simulations of matched nanocontacts to investigate the nature of contact on the atomic scale. Together, these tests show that the ballistic transport equations under-predict the contact size by more than an order of magnitude. The measurements suggest that the low conductance of the contact cannot be explained by the scattering of electrons at defects nor by patchy contact due to surface roughness; instead, the lower-than-expected contact conductance is attributed to approximately a monolayer of insulating surface species on the platinum. Surprisingly, the low conductance persists throughout loading and even after significant sliding of the contact in vacuum. We apply tunneling theory and extract best-fit barrier parameters that describe the properties of this surface layer. The implications of this investigation are that electron transport in device-relevant platinum nanocontacts can be significantly limited by the presence and persistence of surface species, resulting in current flow that is better described by tunneling theory than ballistic electron transport, even for cleaned pure-platinum surfaces and even after loading and sliding in vacuum. read less

Abstract: Metal nanocontacts play a critical role in atomic force microscopy, functional nanostructures, metallic nanoparticles, and nanoscale electromechanical devices. In all cases, knowledge of the area of contact, and its variation with load, is critical for the quantitative prediction of behavior. Often, the contact area is predicted using continuum mechanics models which relate contact size to geometry, material properties, and load. Here we show for platinum nanoprobes that the contact size deviates significantly from these continuum predictions, even at low applied loads and in the absence of irreversible shape change. We use in situ transmission electron microscopy (TEM) with matched molecular dynamics (MD) simulations to investigate the load-dependent size of the contact. Direct measurements of contact radius from MD and TEM exceed the predictions of continuum mechanics by 24%–164%, depending on the model applied. The physical mechanism for this deviation is found to be dislocation activity in the near-surface material, which is fully reversed upon unloading. These findings demonstrate that contact mechanics models are insufficient for predicting contact area in real-world platinum nanostructures, even at ultra-low applied loads. read less

Abstract: Materials with a negative Poisson's ratio (auxetics) are counter intuitive because their mechanical response is unusual. On the other hand, instabilities are usually regarded as deleterious phenomena and thus their prevention is needed. Here, numerical and theoretical evidences have been provided to show that two different elastic instabilities are, rather than deleterious, useful phenomena that cause auxeticity. It has been shown that a negative Poisson's ratio can be found in some face‐centered cubic (FCC) single crystals at a finite strain as they are under uniaxial stress along the [100]‐direction. The auxeticity is associated with a phase transformation induced by the Born–Hill's elastic instability, i.e., an elastic material instability. In addition, it has been found that periodic metallic porous structures can also show a negative Poisson's ratio at finite compressive strain. In this case, buckling of the micro‐structure of the porous structures, which is an elastic and geometric instability, is respondent for the auxeticity. read less

Abstract: In crystallization, nanoparticle aggregation often leads to the formation of orderly structures, even single crystals. Why can nanoparticles form orderly structures and what is the mechanism dominating their orderly aggregation? These questions raise interesting research problems, but the occurrences that could answer them often fail to be directly observed, since the interaction among particles is invisible. Here, we report an attempt to discover the interaction and aggregation of building blocks through a computer simulation, focusing on the shape effect of building blocks on the aggregation. Four types of silver building blocks were selected, each consisting of (100) and (111) facets, but the ratio of these two facets was different. It was found that the area of facets played an important role in selecting the aggregation mode. The facets with a large area and high energy had a high possibility of aggregation. In addition, the effects of solvent viscosity and temperature were also investigated. High viscosity and low temperature enhanced the orderliness of aggregation. This paper reports a detailed view of the aggregation process of silver nanoparticles, which is expected to be helpful in understanding the structure evolution of materials in nonclassical crystallization. read less

Abstract: Understanding the underlying mechanism of crystal nucleation is a fundamental aspect in the prediction and control of materials properties. Classical nucleation theory (CNT) assumes that homogeneous nucleation occurs via random fluctuations within the supercooled liquid, that the structure of the growing clusters resembles the most stable bulk phase, and that the nucleus size is the sole reaction coordinate (RC) of the process. Many materials are, however, known to exhibit multiple steps during crystallization, forming different polymorphs. As a consequence, more complex RCs are often required to capture all relevant information about the process. Here, we employ transition path sampling together with a maximum likelihood analysis of candidate order parameters to identify suitable RCs for the nucleation mechanism during solidification in Ni. In contrast to CNT, the analysis of the reweighted path ensemble shows that a prestructured liquid region that surrounds the crystal cluster is a relevant order parameter that enhances the RC and therefore plays a key role in the description of the nucleus and the interfacial free energy. We demonstrate that prestructured liquid clusters that emerge within the liquid act as precursors of the crystallization in a nonclassical two-step mechanism, which predetermines the coordination of the selected polymorphs. read less

Abstract: Elastic deformation behavior and microstructure evolution of single crystal nickel nanowire in tensile test have been investigated using molecular dynamics simulations. When the temperature is above 300 K, the deformation of nanowire is inhomogeneous in the elastic stage. It can generate a kind of (body‐centered cubic)‐like structure within the material to guide the elastic deformation, rather than the uniform deformation of all the atoms. The (body‐centered cubic)‐like structure will transform to be a body‐centered cubic structure gradually with the increasing of strain and will revert to be a face‐centered cubic structure when the twin crystal appears. The formation of (body‐centered cubic)‐like structure could reduce the density of the material to resist deformation. Besides, the (body‐centered cubic)‐like structure increase with increasing of strain or temperature, and they will gather together to reduce the interface energy. Our conclusion also can be proved by the radial distribution functions g(r). read less

Abstract: Stress concentration around nanosized defects such as cavities always leads to plastic deformation and failure of solids. We investigate the effects of depth, size, and shape of a lotus-type nanocavity on onset plasticity of single crystal Al during nanoindentation on a (001) surface using a quasicontinuum method. The results show that the presence of a nanocavity can greatly affect the contact stiffness (Sc) and yield stress (σy) of the matrix during nanoindentation. For a circular cavity, the Sc and σy gradually increase with the cavity depth. A critical depth can be identified, over which the Sc and σy are insensitive to the cavity depth and it is firstly observed that the nucleated dislocations extend into the matrix and form a y-shaped structure. Moreover, the critical depth varies approximately linearly with the indenter size, regarding the same cavity. The Sc almost linearly decreases with the cavity diameter, while the σy is slightly affected. For an ellipsoidal cavity, the Sc and σy increase with the aspect ratio (AR), while they are less affected when the AR is over 1. Our results shed light in the mechanical behavior of metals with cavities and could also be helpful in designing porous materials and structures. read less

Abstract: The roles of interfaces and matrix grain size in the deformation and failure of polycrystalline Cu-graphene nanolayered (PCuGNL) composites under shear loading are explored with molecular dynamics simulations for different repeat layer spacings (λ), Cu grain sizes (D) and graphene chiralities, and an analytical model is proposed to describe the shear behavior. At the yield stage, the yield stress of the PCuGNL composites is mainly controlled by λ for λ ≤ 15 nm but mainly by D for λ > 15 nm; the yield strain of the composites is approximately a constant value of 0.056, weakly dependent on λ, D and graphene chirality. The shear failure strain and failure stress are determined only by the Cu-graphene interfaces. Small λ reduces the stability of the composites, while large λ decreases their shear failure strength. Considering the yield, failure and interface stability, the optimum λ value for the PCuGNL composites is 2-15 nm. In this optimum λ range, PCuGNL composites can be designed by tailoring Cu-graphene interfaces, regardless of the microstructures of polycrystalline Cu. read less

Abstract: Recent irradiation experiments on concentrated random solid solution alloys (CSAs) show that some CSAs can undergo disorder-to-order transition, i.e., the atoms that are initially randomly distributed on a face centered cubic crystal lattice undergo ordering (e.g., L10 or L12) due to irradiation. In this work, we elucidate that the atomic structure could affect the segregation properties of grain boundaries. While working on Ni and Ni-Fe alloys, from static atomistic simulations on 138 grain boundaries, we show that despite identical alloy composition, Cr segregation is higher in the disordered structures compared to ordered structures in both Ni0.50Fe0.50 and Ni0.75Fe0.25 systems. We also show that grain boundary (GB) energy could act as a descriptor for impurity segregation. We illustrate that there is a direct correlation between Cr segregation and grain boundary energy, i.e., segregation increases with the increase in the GB energy. Such correlation is observed in pure Ni and in the Ni-Fe alloys studied in this work.Recent irradiation experiments on concentrated random solid solution alloys (CSAs) show that some CSAs can undergo disorder-to-order transition, i.e., the atoms that are initially randomly distributed on a face centered cubic crystal lattice undergo ordering (e.g., L10 or L12) due to irradiation. In this work, we elucidate that the atomic structure could affect the segregation properties of grain boundaries. While working on Ni and Ni-Fe alloys, from static atomistic simulations on 138 grain boundaries, we show that despite identical alloy composition, Cr segregation is higher in the disordered structures compared to ordered structures in both Ni0.50Fe0.50 and Ni0.75Fe0.25 systems. We also show that grain boundary (GB) energy could act as a descriptor for impurity segregation. We illustrate that there is a direct correlation between Cr segregation and grain boundary energy, i.e., segregation increases with the increase in the GB energy. Such correlation is observed in pure Ni and in the Ni-Fe alloys studi... read less

Abstract: The SHI irradiations were performed under the CommonUse Facility Program of JAEA. H.A. was supported by JSPSKAKENHI Grant No. 18K04898. I.S., V.J., F.D., and K.N. gratefully acknowledge financial support from the Academy of Finland MESIOS and NANOIS projects, and CPU capacity grants from the IT Centre for Science CSC in Espoo, Finland. Part of this work was performed at the SAXS/WAXS beamline at the Australian Synchrotron, part of ANSTO. P.K. acknowledges the Australian Research Council for financial support. read less

Abstract: As a new class of two-dimensional (2D) materials, 2D covalent organic frameworks (COFs) are proven to possess remarkable electronic and magnetic properties. However, their mechanical behaviours remain almost unexplored. In this work, taking the recently synthesised dimethylmethylene-bridged triphenylamine (DTPA) sheet as an example, we investigate the mechanical behaviours of 2D COFs based on molecular dynamics simulations together with density functional theory calculations. A novel phase transformation is observed in DTPA sheets when a relatively large in-plane compressive strain is applied to them. Specifically, the crystal structures of the transformed phases are topographically different when the compressive loading is applied in different directions. The compression-induced phase transformation in DTPA sheets is attributed to the buckling of their kagome lattice structures and is found to have significant impacts on their material properties. After the phase transformation, Young's modulus, band gap and thermal conductivity of DTPA sheets are greatly reduced and become strongly anisotropic. Moreover, a large in-plane negative Poisson's ratio is found in the transformed phases of DTPA sheets. It is expected that the results of the compression-induced phase transformation and its influence on the material properties observed in the present DTPA sheets can be further extended to other 2D COFs, since most 2D COFs are found to possess a similar kagome lattice structure. read less

Abstract: Liquid Assisted Laser Beam Micromachining (LA-LBMM) process is an advanced machining process that can overcome the limitations of traditional laser beam machining processes. This research involves the use of a Molecular Dynamics (MD) simulation technique to investigate the complex and dynamic mechanisms involved in the LA-LBMM process both in static and dynamic mode. The results of the MD simulation are compared with those of Laser Beam Micromachining (LBMM) performed in air. The study revealed that machining during LA-LBMM process showed higher removal compared with LBMM process. The LA-LBMM process in dynamic mode showed lesser material removal compared with the static mode as the flowing water carrying the heat away from the machining zone. Investigation of the material removal mechanism revealed the presence of a thermal blanket and a bubble formation in the LA-LBMM process, aiding in higher material removal. The findings of this study provide further insights to strengthen the knowledge base of laser beam micromachining technology. read less

Abstract: The mechanical properties of nanoparticles cannot be reliably described by bulk material characteristics due to their atomic structure, leading to pronounced anisotropic behavior. By means of molecular dynamics simulations, we study the impact of 5-nm Ag particles on an adhesive rigid wall. We show that the main characteristics of the impact such as the coefficient of normal restitution, the sticking probability, the maximal contact force, and the degree of plastic deformation of the particle depend sensitively on the angular orientation of the nanoparticle prior to the impact. We introduce the scalar parameter Ω describing the orientation and show that the impact characteristics can be described as functions of Ω. read less

Abstract: Single atom detection is of key importance to solving a wide range of scientific and technological problems. The strong interaction of electrons with matter makes transmission electron microscopy one of the most promising techniques. In particular, aberration correction using scanning transmission electron microscopy has made a significant step forward toward detecting single atoms. However, to overcome radiation damage, related to the use of high-energy electrons, the incoming electron dose should be kept low enough. This results in images exhibiting a low signal-to-noise ratio and extremely weak contrast, especially for light-element nanomaterials. To overcome this problem, a combination of physics-based model fitting and the use of a model-order selection method is proposed, enabling one to detect single atoms with high reliability. read less

Abstract: It is very interesting to discover the elastic properties of engineering material palladium, especially its elastic anisotropy along Hugoniot states. We here investigate the evolution of its high pressure and temperature (PT) elastic ansotropy along Hugoniot using molecular dynamics simulations based on accurate classical interatomic potential. In order to testify the validity of the interatomic potential of Pd in describing the high PT elastic properties, we calculate its isothermal and adiabatic elastic moduli using molecular dynamics method. The obtained data are in good agreement with experimental data. From the isothermal elastic constants, we deduce the Hugoniot acoustic velocities and find that the resulting data are in good agreement with experimental acoustic velocity data. Based on the reliable elastic constants, we further investigate the spacial elastic ansotropy along Hugoniot PT states. It is found that the spacial elastic anisotropy of Pd increases along Hugoniot states. read less

Abstract: We present an embedded atom method (EAM) potential for the binary Cu–Au system. The unary phases are described by two well-tested unary EAM potentials for Cu and Au. We fitted the interaction between Cu and Au to experimental properties of the binary intermetallic phases Cu3Au, CuAu and CuAu3. Particular attention has been paid to reproducing stacking fault energies in order to obtain a potential suitable for studying deformation in this binary system. The resulting energies, lattice constant, elastic properties and melting points are in good agreement with available experimental data. We use nested sampling to show that our potential reproduces the phase boundaries between intermetallic phases and the disordered face-centered cubic solid solution. We benchmark our potential against four popular Cu–Au EAM parameterizations and density-functional theory calculations. read less

Abstract: Department of Chemistry, CDMF, Federal University of S~ao Carlos, P.O. Box 676, S~ao Carlos 13565-905, Brazil Department of Physical Chemistry, University of Valencia, Burjassot 46100, Spain Faculdade de Ciências Exatas e Tecnol ogicas, Universidade Federal da Grande Dourados, Unidade II, CP 533, 79804-970, Dourados, Mato Grosso do Sul, Brazil Department of Condensed Matter Physics, Institute of Physics ‘Gleb Wataghin’, State University of Campinas, 13083-970, Campinas, S~ao Paulo, Brazil Department of Physical Chemistry, Institute of Chemistry, State University of Campinas, 13083-970, Campinas, S~ao Paulo, Brazil Department of Biotechnology and Exact Sciences, Federal University of Tocantins, 77410-530, Gurupi, Tocantins, Brazil read less

Abstract: We present classical potential molecular dynamics simulations of nanoporous gold (np-Au) impacted by a spherical indenter. The atomic structure was generated using a phase field model as a template. In agreement with previous experiments, we observe densification in the region under the indenter. The hardness values obtained from our simulations exhibit a transition from an initially perfect-plastic plateau to hardening behavior in the later stages of indentation. This transition occurs when the relative density beneath the indenter exceeds ∼0.9. Hardness values obtained from the nanoindentation simulations reach 0.6 GPa, due to the densification of the material under the indenter. Elevated dislocation densities are observed in the densified region. The mechanism of pore collapse in the densified layer under the indenter is seen to switch from uniaxial to triaxial, consistent with a change in deformation mechanism from one based on shearing of individual ligaments in np-Au to one involving dislocation-mediated plasticity around voids in a Au single crystal undergoing uniaxial compression. read less

Abstract: Herein, hierarchical dendrite-like Pt crystals with a distinct morphology were synthesized via a facile one-pot method without any templates. Formation of this hierarchical structure is dependent on the reaction duration. Interestingly, different hierarchical structures show different catalytic activities. After a 12 hour reaction, tertiary structures of Pt are formed, which can act as outstanding catalysts in the hydrogen evolution reaction (HER). The onset potential of this dendrite-like Pt catalyst for the HER in a 0.5 M H2SO4 solution is 15 mV, which outperforms that of commercial Pt/C (30 mV). Moreover, it shows significantly improved stability for HER as the polarization curve after 10 000 cycles retains a similar performance as in the initial test; this results in a loss of only 2.6% of its initial current density at an overpotential of 0.05 V. The distinct hierarchical dendrite-like structures are maintained after cycling and current–time tests, which can be responsible for the excellent performance of this catalyst. read less

Abstract: In gas phase synthesis systems, clusters form and grow via condensation, in which a monomer binds to an existing cluster. While a hard-sphere equation is frequently used to predict the condensation rate coefficient, this equation neglects the influences of potential interactions and cluster internal energy on the condensation process. Here, we present a collision rate theory-molecular dynamics simulation approach to calculate condensation probabilities and condensation rate coefficients. We use this approach to examine atomic condensation onto 6-56-atom Au and Mg clusters. The probability of condensation depends upon the initial relative velocity (v) between atom and cluster and the initial impact parameter (b). In all cases, there is a well-defined region of b-v space where condensation is highly probable, and outside of which the condensation probability drops to zero. For Au clusters with more than 10 atoms, we find that at gas temperatures in the 300-1200 K range, the condensation rate coefficient exceeds the hard-sphere rate coefficient by a factor of 1.5-2.0. Conversely, for Au clusters with 10 or fewer atoms and for 14- and 28-atom Mg clusters, as cluster equilibration temperature increases, the condensation rate coefficient drops to values below the hard-sphere rate coefficient. Calculations also yield the self-dissociation rate coefficient, which is found to vary considerably with gas temperature. Finally, calculations results reveal that grazing (high b) atom-cluster collisions at elevated velocity (>1000 m s-1) can result in the colliding atom rebounding (bounce) from the cluster surface or binding while another atom dissociates (replacement). The presented method can be applied in developing rate equations to predict material formation and growth rates in vapor phase systems. read less

Abstract: The sintering of metal nanoparticles (NPs) has been widely studied in the field of nanotechnology, and low-temperature sintering has become the industry standard. In this study, a molecular dynamics (MD) model was established to study the sintering behaviour of silver NPs during low-temperature thermo-compression. Primarily, we studied the sintering process, in which the ratio of neck radius to particle radius (x/r) changes. Under a uniaxial pressure, the maximum ratio in the temperature range 420–425 K was 1. According to the change of x/r, the process can be broken down into three stages: the neck-formation stage, neck-growth stage, and neck-stability stage. In addition, the relationship between potential energy, internal stress, and dislocation density during sintering is discussed. The results showed that cycling internal stress played an important role in sintering. Under the uniaxial pressure, the stress-dislocation interaction was found to be the major mechanism for thermo-compression sintering bec... read less

Abstract: ABSTRACT Molecular dynamics simulation was used to verify a speculation of the existence of a certain face-centred cubic (FCC) to body-centred cubic (BCC) phase transformation pathway. Four FCC metals, Ni, Cu, Au and Ag, were stretched along the [1 0 0] direction at various strain rates and temperatures. Under high strain rate and low temperature, and beyond the elastic limit, the bifurcation of the FCC phase occurred with sudden contraction along one lateral direction and expansion along the other lateral direction. When the lattice constant along the expansion direction converged with that of the stretched direction, the FCC phase transformed into an unstressed BCC phase. By reducing the strain rate or increasing the temperature, dislocation or ‘momentum-induced melting’ mechanisms began to control the plastic deformation of the FCC metals, respectively. read less

Abstract: Low-cycle fatigue behaviors of graphene-copper nanolayered (GCuNL) composites are explored at different interface configurations and repeat layer spacings. The graphene interfaces can trap dislocations through impeding the propagation of dislocations in copper layers, giving rise to the absence of softening, and an increase in the fatigue strength of GCuNL composites (up to 400% that of pure copper). This anti-fatigue effect is independent of the crystal orientation of copper or the chirality of graphene due to interfacial constraints and can be controlled by tailoring the repeat layer spacing. Low repeat layer spacing increases the instability and nonlinearity of the composites, while high repeat layer spacing decreases the anti-fatigue effect. The optimum value of the repeat layer spacing for the GCuNL composites is 3-7 nm, in order to achieve a balanced anti-fatigue capability and interface stability. read less

Abstract: The potential for explosive cathode emission due to nanoprotrusions subjected to Maxwell stress and heating from strong electric fields is probed self-consistently based on non-equilibrium molecular-dynamics. The focus is on determining the electric field magnitudes that could lead to material ejection, assessing dependencies of the instability on the nanoprotrusion height and cross-sectional area, and the role of time-dependent thermal conductivity and local temperature changes. Our results indicate that large aspect ratios would facilitate mass ejection, with protrusion break up occurring over times in the 25 ns range, in agreement with experimental reports on explosive emission. read less

Abstract: The incidence of energetic laser pulses on a metal foam may lead to foam ablation. The processes occurring in the foam may differ strongly from those in a bulk metal: The absorption of laser light, energy transfer to the atomic system, heat conduction, and finally, the atomistic processes—such as melting or evaporation—may be different. In addition, novel phenomena take place, such as a reorganization of the ligament network in the foam. We study all these processes in an Au foam of average porosity 79% and an average ligament diameter of 2.5 nm, using molecular dynamics simulation. The coupling of the electronic system to the atomic system is modeled by using the electron–phonon coupling, g, and the electronic heat diffusivity, κe, as model parameters, since their actual values for foams are unknown. We show that the foam coarsens under laser irradiation. While κe governs the homogeneity of the processes, g mainly determines their time scale. The final porosity reached is independent of the value of g. read less

Abstract: A silver nanoparticle sintering method is promising for the fabrication of flexible electronics. An ultrasonic-assisted sintering process is presented to obtain conductive patterns with low resistivity on a paper-based substrate. Compared to the conventional hot-pressing sintering process, the proposed method can efficiently fabricate densified microstructures with better electrical properties at low temperature and pressure. In particular, the effects of ultrasonic effective time, entry time, and amplitude were systematically analyzed. It was found that lower resistivity preferred larger ultrasonic amplitude. The ultrasonic effective time happened during the first 3 min of the entire sintering process, which implied that the ultrasonic synergetic effect occurred at the very beginning stage of the sintering process. Additionally, better conductivity of the sintered patterns was obtained when the ultrasonic entry time was shorter. The mechanism of the synergetic effect of ultrasonic sintering was also disc... read less

Abstract: The plastic deformation mechanism of Cu/Ag multilayers is investigated by molecular dynamics (MD) simulation in a nanoindentation process. The result shows that due to the interface barrier, the dislocations pile-up at the interface and then the plastic deformation of the Ag matrix occurs due to the nucleation and emission of dislocations from the interface and the dislocation propagation through the interface. In addition, it is found that the incipient plastic deformation of Cu/Ag multilayers is postponed, compared with that of bulk single-crystal Cu. The plastic deformation of Cu/Ag multilayers is affected by the lattice mismatch more than by the difference in stacking fault energy (SFE) between Cu and Ag. The dislocation pile-up at the interface is determined by the obstruction of the mismatch dislocation network and the attraction of the image force. Furthermore, this work provides a basis for further understanding and tailoring metal multilayers with good mechanical properties, which may facilitate the design and development of multilayer materials with low cost production strategies. read less

Abstract: A study has been presented on the effects of intrinsic mechanical parameters, such as surface stress, surface elastic modulus, surface porosity, permeability and grain size on the corrosion failure of nanocomposite coatings. A set of mechano-electrochemical equations was developed by combining the popular Butler–Volmer and Duhem expressions to analyze the direct influence of mechanical parameters on the electrochemical reactions in nanocomposite coatings. Nanocomposite coatings of Ni with Al2O3, SiC, ZrO2 and Graphene nanoparticles were studied as examples. The predictions showed that the corrosion rate of the nanocoatings increased with increasing grain size due to increase in surface stress, surface porosity and permeability of nanocoatings. A detailed experimental study was performed in which the nanocomposite coatings were subjected to an accelerated corrosion testing. The experimental results helped to develop and validate the equations by qualitative comparison between the experimental and predicted results showing good agreement between the two. read less

Abstract: The structure of bimetallic NiCu nanoparticles (NP) is investigated as a function of their composition and size by means of Lattice MonteCarlo (LMC) and molecular dynamics (MD) simulations. According to our results, copper segregation takes place at any composition of the particles. We found that this feature is not size-dependent. In contrast, nickel segregation depends on the NP size. When the size increases, Ni atoms tend to remain in the vicinity of the surface and deeper. For smaller NPs, Ni atoms are located at the surface as well. Our results also showed that most of the metal atoms segregated at the surface area were found to decorate edges and/or form islands. Our findings agree qualitatively with the experimental data found in the literature. In addition, we comment on the reactivity of these nanoparticles. read less

Abstract: We present a theoretical investigation of the structural and vibrational properties of the K/Cu(110) adsorbed systems using the embedded atom method. The surface relaxation, phonon dispersions, and polarization of vibrational modes as well as the local density of states have been calculated as a function of potassium coverage on reconstructed and unreconstructed Cu(110) surface. On the reconstructed surface, two K-substrate stretching modes have been found, which do not show any coverage dependence. The obtained vibrational frequencies of surface states are in close agreement with available experimental data. read less

Abstract: Three-dimensional non-equilibrium molecular dynamics simulations are performed to investigate the tribological characteristics of polycrystalline Ni-Ni and Ni-Cu high-speed sliding systems. The grain size distribution, temperature profiles and heat density changes are obtained. Founded on simulation results a correlation between surface grain size and heat conduction is proposed which can help us to fully understand the mechanism of polycrystalline friction behavior and design better friction material. This interaction shows that the sizes of the grains located in the interface decrease and it increases the resistance of friction heat conduction during sliding. read less

Abstract: Molecular dynamics (MD) simulations under different mechanical and thermal constraints are carried out with a nanovoid embedded inside a single-crystal, face-centred-cubic copper. The dislocation emission angles measured from MD plots under 0.1 K, uniaxial-strain simulation are in line with the theoretical model. The dislocation density calculated from simulation is qualitatively consistent with the experimental measurement in terms of a saturation feature. The ‘relatively farthest-travelled’ atoms are employed to reflect the correlation between the dislocation structure and the void growth. At a smaller scale, the incomplete shear dislocation loops on the slip plane contribute to the local material transport. At a larger scale, the dislocation structures formed by those incomplete shear loops further facilitate the growth of nanovoid. Compared to the uniaxial-strain case, the void growth under the uniaxial-stress is very limited. The uniaxial-strain loading results in an octahedron void shape. The uniaxial-stress loading turns the nanovoid into a prolate ellipsoid along the loading direction. In the simulation, the largest specimen contains 12 million atoms and the lowest strain rate applied is 2 × 106 s−1. Under all the different thermomechanical constraints concerned, the formation of incomplete shear dislocation loops are found capable of growing the void. read less

Abstract: The molecular dynamics (MD) simulation of Ni/Cu multilayers under a high speed grinding process with a diamond tip is performed, with the aim of investigating the effects of varying machining parameters on subsurface damage and material removal in the Ni/Cu multilayers. A series of key factors, consisting of grinding speed, tool radius and depth of cut, that influence the deformation of the workpiece are systemically studied in terms of surface morphology, dislocation movement, grinding temperature and average grinding force. Both the grinding temperature and force increase with increasing grinding speed, tool radius and depth of cut. In addition, a relatively small grinding velocity results in more stacking faults (SF) and a greater volume of material pileups on the sides of the groove. A good surface integrity of the Ni/Cu multilayers, with relatively fewer lattice defects, is more easily obtained by a machining process with a smaller tool radius or cutting depth. The results also show that the grinding temperature of Ni/Cu multilayers with varying grinding speeds, tool radii and cutting depths is higher than that of pure Ni thin film. read less

Abstract: Recent experiments show that the grain boundaries (GBs) of copper nanoparticles (NPs) lead to an outstanding performance in reducing CO2 and CO to alcohol products. We report here multiscale simulations that simulate experimental synthesis conditions to predict the structure of a 10 nm Cu NP (158 555 atoms). To identify active sites, we first predict the CO binding at a large number of sites and select four exhibiting CO binding stronger than the (211) step surface. Then, we predict the formation energy of the *OCCOH intermediate as a descriptor for C-C coupling, identifying two active sites, both of which have an under-coordinated surface square site adjacent to a subsurface stacking fault. We then propose a periodic Cu surface (4 by 4 supercell) with a similar site that substantially decreases the formation energy of *OCCOH, by 0.14 eV. read less

Abstract: Most materials expand upon heating because the coefficient of thermal expansion (CTE), the fundamental property of materials characterizing the mechanical response of the materials to heating, is positive. There have been some reports of materials that exhibit negative thermal expansion (NTE), but most of these have been in complex alloys, where NTE originates from the transverse vibrations of the materials. Here, we show using molecular dynamics simulations that some single crystal monatomic FCC metal nanowires can exhibit NTE along the length direction due to a novel thermomechanical coupling. We develop an analytic model for the CTE in nanowires that is a function of the surface stress, elastic modulus, and nanowire size. The model suggests that the CTE of nanowires can be reduced due to elastic softening of the materials and also due to surface stress. For the nanowires, the model predicts that the CTE reduction can lead to NTE if the nanowire Young's modulus is sufficiently reduced while the nanowire surface stress remains sufficiently large, which is in excellent agreement with the molecular dynamics simulation results. Overall, we find a "smaller is smaller" trend for the CTE of nanowires, leading to this unexpected, surface-stress-driven mechanism for NTE in nanoscale materials. read less

Abstract: Some metal alloys subjected to irreversible plastic deformation can repair the inflicted damage and/or recover their original shape upon heating. The conventional shape memory effect in metallic alloys relies on collective, or “military” phase transformations. This work demonstrates a new and fundamentally different type of self‐healing and shape memory in single crystalline faceted nano and microparticles of pure gold, which are plastically deformed with an atomic force microscope tip. It is shown that annealing of the deformed particles at elevated temperatures leads to nearly full recovery of their initial asymmetric polyhedral shape, which does not correspond to global energy minimum shape. The atomistic molecular dynamic simulations demonstrate that the shape recovery of the particles is controlled by the self‐diffusion of gold atoms along the terrace ledges formed during the particles indentation. This ledge‐guided diffusion leads to shape recovery by the irreversible diffusion process. A semiquantitative model of healing developed in this work demonstrates a good agreement with the experimental data. read less

Abstract: Molecular dynamics simulations have been performed to study the mechanical properties of a columnar nanocrystalline copper with a mean grain size between 9.0 and 24 nm. A melting–cooling method has been used to generate the initial samples: this method produces realistic samples that contain defects inside the grains such as dislocations and vacancies. The results of uniaxial tensile tests applied to these samples reveal the presence of a critical mean grain size between 16 and 20 nm, for which there is an inversion of the conventional Hall–Petch relation. The principal mechanisms of deformation present in the samples correspond to a combination of dislocations and grain boundary sliding. In addition, this analysis shows the presence of sliding planes generated by the motion of perfect edge dislocations that are absorbed by grain boundaries. It is the initial defects present inside the grains that lead to this mechanism of deformation. An analysis of the atomic configurations further shows that nucleation and propagation of cracks are localised on the grain boundaries especially on the triple grains junctions. read less

Abstract: The ability of short pulse laser ablation in liquids to produce clean colloidal nanoparticles and unusual surface morphology has been employed in a broad range of practical applications. In this paper, we report the results of large-scale molecular dynamics simulations aimed at revealing the key processes that control the surface morphology and nanoparticle size distributions by pulsed laser ablation in liquids. The simulations of bulk Ag targets irradiated in water are performed with an advanced computational model combining a coarse-grained representation of liquid environment and an atomistic description of laser interaction with metal targets. For the irradiation conditions that correspond to the spallation regime in vacuum, the simulations predict that the water environment can prevent the complete separation of the spalled layer from the target, leading to the formation of large subsurface voids stabilized by rapid cooling and solidification. The subsequent irradiation of the laser-modified surface is found to result in a more efficient ablation and nanoparticle generation, thus suggesting the possibility of the incubation effect in multipulse laser ablation in liquids. The simulations performed at higher laser fluences that correspond to the phase explosion regime in vacuum reveal the accumulation of the ablation plume at the interface with the water environment and the formation of a hot metal layer. The water in contact with the metal layer is brought to the supercritical state and provides an environment suitable for nucleation and growth of small metal nanoparticles from metal atoms emitted from the hot metal layer. The metal layer itself has limited stability and can readily disintegrate into large (tens of nanometers) nanoparticles. The layer disintegration is facilitated by the Rayleigh–Taylor instability of the interface between the higher density metal layer decelerated by the pressure from the lighter supercritical water. The nanoparticles emerging from the layer disintegration are rapidly cooled and solidified due to the interaction with water environment, with a cooling rate of ∼2 × 1012 K/s observed in the simulations. The computational prediction of two distinct mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes provides a plausible explanation for the experimental observations of bimodal nanoparticle size distributions in laser ablation in liquids. The ultrahigh cooling and solidification rates suggest the possibility for generation of nanoparticles featuring metastable phases and highly nonequilibrium structures. read less

Abstract: Nucleation is a key step during crystallization, but a complete understanding of the fundamental atomistic processes remains elusive. We investigate the mechanism of nucleation during solidification in nickel for various undercoolings using transition path sampling simulations. The temperature dependence of the free energy barriers and rate constants that we obtain is consistent with the predictions of classical nucleation theory and experiments. However, our analysis of the transition path ensemble reveals a mechanism that deviates from the classical picture of nucleation: the growing solid clusters have predominantly non-spherical shapes and consist of face-centered-cubic and random hexagonal-close-packed coordinated atoms surrounded by a cloud of prestructured liquid. The nucleation initiates in regions of supercooled liquid that are characterized by a high orientational order with structural features that predetermine the polymorph selection. These results provide atomistic insight not only into the nucleation mechanism of nickel but also into the role of the preordered liquid regions as precursors for crystallization. read less

Abstract: A molecular dynamics (MD) model is used to study the potential for mass ejection from a metal nanoprotrusion, driven by high fields and temperature increases. Three-dimensional calculations of the electric fields surrounding the metal emitter are used to obtain the Maxwell stress on the metal. This surface loading is coupled into MD simulations. Our results show that mass ejection from the nanotip is possible and indicate that both larger aspect ratios and higher local temperatures will drive the instability. Hence it is predicted that in a nonuniform distribution of emitters, the longer and thinner sites will suffer the most damage, which is generally in keeping with the trends of a recent experimental report (Parson et al 2014 IEEE Trans. Plasma Sci. 42 3982). A possible hypothesis for mass ejection in the absence of a distinct nanoprotrusion is also discussed. read less

Abstract: Glycerol, which is one of the most abundant by-products in biodiesel production, can be converted into H2 through aqueous-phase reforming (APR). Dehydrogenation is one of the main processes in glycerol APR. In this work, we use computational methods to study Pt(1 1 1)-catalysed glycerol dehydrogenation under aqueous conditions. There are 84 intermediates and 250 possible reactions in the dehydrogenation network. Inclusion of the liquid environment adds computational expense, especially if we are to study all the reaction intermediates and reactions under explicit water solvation using quantum methods. In this work, we present a method that can be used to efficiently estimate reaction energies under explicit solvation with reasonable accuracy and computational expense. The method couples a linear scaling relationship for obtaining adsorbate binding energies with Lennard-Jones + Coulomb potentials for obtaining water–adsorbate interaction energies. Comparing reaction energies calculated with this approach to reaction energies obtained from a more extensive approach that attains quantum-level accuracy (published previously by our group), we find good correlation (R2 = 0.84) and reasonable accuracy (the mean absolute error, MAE = 0.28 eV). read less

Abstract: Picric acid (PA) is a severe environmental and security risk due to its unstable, toxic, and explosive properties. It is also challenging to detect in trace amounts and in situ because of its highly acidic and anionic character. Here, we assess sensing of PA under nonlaboratory conditions using surface-enhanced Raman scattering (SERS) silver nanopillar substrates and hand-held Raman spectroscopy equipment. The advancing elasto-capillarity effects are explained by molecular dynamics simulations. We obtain a SERS PA detection limit on the order of 20 ppt, corresponding attomole amounts, which together with the simple analysis methodology demonstrates that the presented approach is highly competitive for ultrasensitive analysis in the field. read less

Abstract: Smoothed molecular dynamics (SMD) method is a recently proposed efficient molecular simulation method by introducing one set of background mesh and mapping process into molecular dynamics (MD) flow chart. SMD can sharply enlarge MD time step size while maintaining global accuracy. MD‐SMD coupling method was proposed to improve the capability to describe local atom disorders. The coupling method is greatly improved in this paper in two essential aspects. Firstly, a transition scheme is proposed to avoid artificial wave reflection at the interface of MD and SMD regions. The new transition scheme has simple formulation and high efficiency, and the wave reflection can be well suppressed. Secondly, an adaptive scheme is proposed to automatically identify the regions requiring MD simulation. Two adaptive criteria, the centro‐symmetry parameter criterion and the displacement criterion, are also proposed. It is found that both the two criteria can achieve good accuracy but the efficiency of the displacement criterion is much better. The coupling method does not demand reduction in mesh size near the interface, and a multiple time stepping scheme is adopted to ensure high efficiency. Numerical results including wave propagation, nano‐indentation, and crack propagation validate the method and show nice accuracy. Copyright © 2016 John Wiley & Sons, Ltd. read less

Abstract: The growth of Pb ultrathin films on Cu(111) has been long studied in connection with electronic quantum-size effects and for the different temperature-dependent growth kinetics. At low temperature the formation of a wetting layer (1 monolayer (ML)), is followed by an instability of the 2 ML film and a regular layer-by-layer growth is then only observed for more than two monolayers. The 2 ML film was, however, shown to be stabilized by alloying Pb with 20% Tl. This work presents a theoretical study of the dynamics of the wetting layer as well as for 2 ML Pb0.8Tl0.2, 3 and 4 ML Pb on Cu(111) in the 4 × 4 commensurate phase, for which detailed inelastic Helium atom scattering (HAS) spectra have been measured. The present calculations based on the embedded atom method (EAM) include the dynamics of the substrate. Besides leading to a detailed interpretation of the HAS experimental data, the present results are compared with a previous density-functional perturbation theory (DFPT) study for 3 to 7 ML Pb on a ri... read less

Abstract: The structures of the Si/Cu heterogenous interface impacted by a nanoindenter with different incident angles and depths are investigated in detail using molecular dynamics simulation. The simulation results suggest that for certain incident angles, the nanoindenter with increasing depth can firstly increase the stress of each atom at the interface and it then introduces more serious structural deformation of the Si/Cu heterogenous interface. A nanoindenter with increasing incident angle (absolute value) can increase the length of the Si or Cu extended atom layer. It is worth mentioning that when the incident angle of the nanoindenter is between −45° and 45°, these Si or Cu atoms near the nanoindenter reach a stable state, which has a lower stress and a shorter length of the Si or Cu extended atom layer than those of the other incident angles. This may give a direction to the planarizing process of very large scale integration circuits manufacture. read less

Abstract: Metallic nanoparticles are usually polyhedrons instead of perfect spheres, which presents a challenge to characterize their elastic response. In the present paper, the elastic compression of truncated octahedral nanoparticles is investigated through finite element calculations and atomic simulations. An analytical expression of load is obtained for octahedral particles, which is linearly proportional to indent depth, instead of the 3/2 power law relation predicted by Hertzian model for elastic sphere. Comparisons with molecular dynamics simulations demonstrate that the obtained relation can predict the elastic response of polyhedral nanoparticles. This study is helpful to measure the elastic properties of polyhedral nanoparticles, and characterize their elastic response. read less

Abstract: Solid films are considered as typical model systems to study size effects on thermal vacancy concentration in nanomaterials. By combining the generalized Young-Laplace equation with the chemical potential of vacancies, a strict size-dependent thermodynamic model of vacancies, which includes the surface intrinsic elastic parameters of the eigenstress, Young's modulus and the geometric size of the solid films, was established. The vacancy concentration changes in the film with respect to the bulk value, depending on the geometric size and surface stress sign of the solid films. Atomistic simulations of Au and Pt films verified the developed thermodynamic model. These results provide physical insights into the size-dependent thermal vacancy concentration in nanomaterials. read less

Abstract: This report employed molecular statics simulation and density‐functional‐theory calculation to study the Poisson's ratios of face‐centered‐cubic materials. We provide numerical and theoretical evidences to show that cubic materials can exhibit auxetic behavior in a principal direction under proper loading conditions. When a stress perpendicular to the loading direction is applied, cubic materials can exhibit a negative Poisson's ratio at finite strain. The negative Poisson's ratio behavior, including its direction and value, is highly dependent on the direction and magnitude of the transversely applied stresses. As a result, we show that it is possible to tune the direction and magnitude of the negative Poisson's ratio behavior of cubic materials by controlling the transverse loadings. read less

Abstract: Interfaces with different structural units have a strong impact on their microscopic deformation behavior and correlated macroscopic mechanical response. In current study, we elucidate the underlying nanoscale friction mechanisms of Cu bicrystals by means of molecular dynamics simulations. Four grain boundaries, i.e., pure tilt and twist with two misorientation angles of 7.63° and 67.38°, are considered to address the grain boundary structure dependence of the friction. While the small- and high-angle tilt grain boundaries are respectively composed of parallel edge dislocation dipoles and edge dislocations of like sign, the small- and high-angle twist ones respectively incorporate two sets of intersecting screw dislocations and a planar defect. Simulation results demonstrate that the grain boundary resistance to dislocation motion and absorption, as well as the grain boundary evolution, are significantly varied with grain boundary structural units. It is found that splitting, annihilation and generation of grain boundary dislocations are the three competing decomposition mechanisms of the well-defined grain boundaries. The anisotropic dislocation-grain boundary interactions in turn results in a strong grain boundary structure dependence of the frictional response for scratching in the vicinity of grain boundaries. These findings will not only advance our understanding of the interface-dependent nanoscale friction behavior of metals, but also provide rational design guidelines for the synthesis of advanced functional nanostructured materials with unique internal interface textures. read less

Abstract: We test the localization model (LM) prediction of a parameter-free relationship between the α-structural relaxation time τ α and the Debye–Waller factor  〈u 2 〉  for a series of simulated glass-forming Cu–Zr metallic liquids having a range of alloy compositions. After validating this relationship between the picosecond (‘fast’) and long-time relaxation dynamics over the full range of temperatures and alloy compositions investigated in our simulations, we show that it is also possible to estimate the self-diffusion coefficients of the individual atomic species (D Cu, D  Zr) and the average diffusion coefficient D using the LM, in conjunction with the empirical fractional Stokes–Einstein (FSE) relation linking these diffusion coefficients to τ α . We further observe that the fragility and extent of decoupling between D and τ α strongly correlate with  〈u 2 〉  at the onset temperature of glass-formation T A where particle caging and the breakdown of Arrhenius relaxation first emerge. read less

Abstract: We present a molecular dynamics simulation scheme that we apply to study the time evolution of the self-organized growth process of metalcluster assemblies formed by sputter-deposited gold atoms on a planar surface. The simulation model incorporates the characteristics of the plasma-assisted deposition process and allows for an investigation over a wide range of deposition parameters. It is used to obtain data for the cluster properties which can directly be compared with recently published experimental data for gold on polystyrene [M. Schwartzkopf et al., ACS Appl. Mater. Interfaces 7, 13547 (2015)]. While good agreement is found between the two, the simulations additionally provide valuable time-dependent real-space data of the surface morphology, some of whose details are hidden in the reciprocal-space scattering images that were used for the experimental analysis. read less

Abstract: The crystallinity of gold nanoparticles during coalescence or sintering is investigated by molecular dynamics. The method is validated by the attainment of the Au melting temperature that increases with increasing particle size approaching the Au melting point. The morphology and crystal dynamics of nanoparticles of (un)equal size during sintering are elucidated. The characteristic sintering time of particle pairs is determined by tracing their surface area evolution during coalescence. The crystallinity is quantified by the disorder variable indicating the system's degree of disorder. The atoms at the grain boundaries are amorphous, especially during particle adhesion and during sintering when grains of different orientation are formed. Initial grain orientation affects final particle morphology leading to exposure of different crystal surfaces that can affect the performance of Au nanoparticles (e.g., catalytic efficiency). Coalescence between crystalline and amorphous nanoparticles of different size results in polycrystalline particles of increasing crystallinity with time and temperature. Crystallinity affects the sintering rate and mechanism. Such simulations of free-standing Au nanoparticle coalescence are relevant also to Au nanoparticles on supports that do not exhibit strong affinity or strong metal support interactions. © 2015 American Institute of Chemical Engineers AIChE J, 62: 589–598, 2016 read less

Abstract: The previously developed bridging cell method for modeling coupled continuum/atomistic systems at finite temperature is used to model fatigue crack growth in single crystal nickel under two crystal orientations at different temperatures. The method is expanded to implement a temperature‐dependent embedded atom method potential for finite temperature simulations avoiding time‐scale restrictions associated with small timesteps. Results for the fatigue simulation were compared with respect to deformation behavior, stress distribution, and crack length. Results showed very different crack growth mechanisms between the two crystal orientations as well as reduced resistance to crack growth with increased temperature. Copyright © 2015 John Wiley & Sons, Ltd. read less

Abstract: By using a molecular dynamics method with EAM potential, we study the evolution phenomena of metal twist grain boundaries (GBs) in the [100], [111] and [110] orientations, together with their bimetal interface, under anticlockwise and clockwise torsions. Our results show that there are different evolution behaviors of the GB screw dislocations for single metals (Al and Ni) and their bimetal interface (Al/Ni) under torsion. Specifically, for the single metals in the [100] and [111] orientations, their GBs evolve toward lower or higher angle twist GBs depending on the twist direction. For Ni in the [110] orientation, the dislocations spread not only in the GB region but also in the grain interior. However, for the bimetal interface, the propagation of dislocations is not only reduced dramatically but also limited to the interface region, showing that there is an inhibition effect. Therefore, such an inhibition effect can enhance the stability of nanomaterials which is very useful for the further design of nanodevices. read less

Abstract: Fundamental factors governing the ion-desorption efficiency and extent of internal-energy transfer to a chemical thermometer, benzylpyridinium ion ([BP]+), generated in the surface-assisted laser desorption/ionization (SALDI) process, were systematically investigated using noble metal nanoparticles (NPs), including AuNPs, AgNPs, PdNPs, and PtNPs, as substrates, with an average particle size of 1.7–3.1 nm in diameter. In the correlation of ion-desorption efficiency and internal-energy transfer with physicochemical properties of the NPs, laser-induced heating of the NPs, which are dependent on their photoabsorption efficiencies, was found to be a key factor in governing the ion-desorption efficiency and the extent of internal-energy transfer. This suggested that the thermal-driven desorption played a significant role in the ion-desorption process. In addition, a stronger binding affinity of [BP]+ to the surface of the NPs could hinder its desorption from the NPs, and this could be another factor in determin... read less

Abstract: We investigate through molecular dynamic simulations the dependence of dislocation creation on tensile orientation in face-centered-cubic ductile metals under high strain rate loading. It is found that while dislocations generally originate from the double-layer defect clusters consisting of flatted octahedral structures (FOSs), the formation mechanism and the types of FOSs, as well as the types of nucleated dislocations, depend on the applied loading directions. For the loading along the [1¯10], [1¯1¯2], and [111] crystal directions, it is shown that a pair of the nearest-neighboring atoms move away to form the elongated FOS. However, for the loading along the [100] crystal direction, a pair of the next-nearest-neighboring atoms move close to form the compressed FOS. According to the uniform deformation amount of the spacing vector for a pair of neighboring atoms and the stress component along the Burgers vector on the stacking fault plane, we analytically predict the activated types of FOSs and dislocat... read less

Abstract: We use large-scale molecular dynamics simulations to characterize the mechanical responses of nanocrystalline bulk and thin-film samples with average grain size ranging from 5 to 40 nm and at two strain rates. Our simulations show Hall–Petch maxima for both yield and flow stresses and for both sets of specimens. We find that the presence of free surface decreases both the yield and flow stresses and, interestingly, the Hall–Petch maximum for slabs occur at a larger grain size than for the bulk samples. A quantitative analysis of plastic slip on grain interiors and boundaries reveals that the shift in the maximum results from a combination of higher intergranular slip and weaker size dependence of dislocation activity in the slabs as compared with the bulk. Finally, increasing strain rate increases both yield and flow stresses and this rate effect is dominated by the plasticity involving full dislocations; plastic slip by partial dislocations and grain boundary processes exhibit weaker size effects. read less

Abstract: In current study, molecular dynamics simulations are performed to investigate nanoimprint processes on single crystal aluminium thin films with silicon stamp. An EAM potential developed for aluminium is adopted to describe the Al-Al interaction in the aluminium thin film, and the Lennard-Jones potential is used to describe the Al-Si atomic interaction. Simulations show that dislocation events are the main plastic deformation mode of Al thin film in nanoimprint. It is found that stamp size and stamp shape have significant influence on the dislocation events and imprint forces during the nanoimprint processes. Keywords-nanoimprint; molecular dynamics; single crystal aluminium; thin film; stamp read less

Abstract: In the present work, we perform experiments and molecular dynamics simulations to elucidate the underlying deformation mechanisms of single crystalline copper under the load-controlled multi-passes nanoscratching using a triangular pyramidal probe. The correlation of microscopic deformation behavior of the material with macroscopically-observed machining results is revealed. Moreover, the influence of crystallographic orientation on the nanoscratching of single crystalline copper is examined. Our simulation results indicate that the plastic deformation of single crystalline Cu under the nanoscratching is exclusively governed by dislocation mechanisms. However, there is no glissile dislocation structure formed due to the probe oscillation under the load-controlled mode. Both experiments and MD simulations demonstrate that the machined surface morphologies in terms of groove depth and surface pile-up exhibit strong crystallographic orientation dependence, because of different geometries of activated slip planes cutting with free surfaces and strain hardening abilities associated with different crystallographic orientations. read less

Abstract: The nanojoining process of Ag-Au hybrid nanowires at 800K was comprehensively studied by virtue of molecular dynamics (MD) simulation. Three kinds of configurations including end-to-end, T-like and X-like were built in the simulation aiming to understand the nanojoining mechanism. The detailed dynamic evolution of atoms, crystal structure transformation and defects development during the nanojoining processes were performed. The results indicate that there are two stages in the nanojoining process of Ag-Au nanowires which are atom diffusion and new bonds formation. Temperature is a key parameter affecting both stages ascribed to the energy supply and the optimum temperature for Ag-Au nanojoint with diameter of 4.08 nm has been discussed. The mechanical properties of the nanojoint were examined with simulation of tensile test on the end-to-end joint. It was revealed that the nanojoint was strong enough to resist fracture at the joining area. read less

Abstract: Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations have been used to investigate the atomic and electronic structure of gold deposited (between 0.8 and 1.0 monolayer) on the Pt(111) face in ultrahigh vacuum at room temperature. The analysis of LEED and STM measurements indicates two-dimensional growth of the first Au monolayer. Change of the measured surface lattice constant equal to 2.80 Å after Au adsorption was not observed. Based on DFT, the distance between the nearest atoms in the case of bare Pt(111) and Au/Pt(111) surface is equal to 2.83 Å, which gives 1% difference in comparison with STM values. The first and second interlayer spacing of the clean Pt(111) surface are expanded by +0.87% and contracted by −0.43%, respectively. The adsorption energy of the Au atom on the Pt(111) surface is dependent on the adsorption position, and there is a preference for a hollow fcc site. For the Au/Pt(111) surface, the top interlayer spacing is expanded by +2.16% with respect to the ideal bulk value. Changes in the electronic properties of the Au/Pt(111) system below the Fermi level connected to the interaction of Au atoms with Pt(111) surface are observed. read less

Abstract: Molecular dynamics simulations were used to model the Eshelby dislocation inside Pd and Ir nanowires and to predict the powder diffraction pattern using the Debye scattering equation. We find that the ideal dislocation solution by Eshelby is in good agreement with the observed twist angle and deviatoric strain, even though it ignores both the splitting of the Eshelby dislocation into two partials and surface stress. Surface stress plays a significant role only for nanorods with small aspect ratio (∼1:1). We also find that Wilson's prediction on the diffraction peak broadening for the Eshelby dislocation is overestimated because it ignores the fact that the Eshelby twist relaxes the deviatoric strain. Moreover, the twist loosens the correlation along the nanorod, causing additional line profile broadening, which is read by diffraction as a decrease of coherent domain size when the total twist angle is bigger than 1.5°. Overall, our findings suggest a novel way to predict and analyze the dislocations as well as the resulting strain fields in the twisted nanocrystalline rods. read less

Abstract: Using equilibrium and non-equilibrium molecular dynamics simulations, we study the flow of argon fluid above the critical temperature in a planar nanochannel delimited by graphene walls. We observe that, as a function of pressure, the slip length first decreases due to the decreasing mean free path of gas molecules, reaches the minimum value when the pressure is close to the critical pressure, and then increases with further increase in pressure. We demonstrate that the slip length increase at high pressures is due to the fact that the viscosity of fluid increases much faster with pressure than the friction coefficient between the fluid and the graphene. This behavior is clearly exhibited in the case of graphene due to a very smooth potential landscape originating from a very high atomic density of graphene planes. By contrast, on surfaces with lower atomic density, such as an (100) Au surface, the slip length for high fluid pressures is essentially zero, regardless of the nature of interaction between fluid and the solid wall. read less

Abstract: Atomistic simulations can be used to compute damping from first principles and gain unprecedented insights into the mechanisms of dissipation. However, the technique is still in its infancy and many foundational aspects remain unexplored. As a step toward addressing these issues, we present here a comparative study of five different methods for estimating damping under isothermal conditions. Classical molecular dynamics was used to simulate the fundamental longitudinal-mode oscillations of nanowires and nanofilms of silicon and nickel at room temperature (300 K) in the canonical ensemble using the Nosé-Hoover thermostat. In the subresonant regime, damping was quantified using the loss tangent and loss factor during steady-state harmonic vibration. The quality factor was obtained by analyzing the spectrum of thermomechanical noise and also from the Duffing-like nonlinearity in the frequency response under harmonic excitation. In addition, the nonlinear logarithmic decrement was obtained from the Hilbert transform of freely decaying oscillations. We discuss the factors that must be considered while selecting simulation parameters, establish criteria for convergence and linearity, and highlight the relative merits and limitations of each method. read less

Abstract: Although we often think about crystalline materials in terms of highly organized arrays of atoms, molecules, or even colloidal particles, many of the important properties of this diverse class of materials relating to their catalytic behavior, thermodynamic stability, and mechanical properties derive from the dynamics and thermodynamics of their interfacial regions, which we find they have a dynamics more like glass-forming (GF) liquids than crystals at elevated temperatures. This is a general problem arising in any attempt to model the properties of naturally occurring crystalline materials since many aspects of the dynamics of glass-forming liquids remain mysterious. We examine the nature of this phenomenon in the "simple" case of the (110) interface of crystalline Ni, based on a standard embedded-atom model potential, and we then quantify the collective dynamics in this interfacial region using newly developed methods for characterizing the cooperative dynamics of glass-forming liquids. As in our former studies of the interfacial dynamics of grain-boundaries and the interfacial dynamics of crystalline Ni nanoparticles (NPs), we find that the interface of bulk crystalline Ni exhibits all the characteristics of glass-forming materials, even at temperatures well below the equilibrium crystal melting temperature, Tm. This perspective offers a new approach to modeling and engineering the properties of crystalline materials. read less

Abstract: Molecular Dynamics (MD) was used to simulate cylindrical Pd and Ir domains with ideal dislocations parallel to the axis. Results show significant discrepancies with respect to predictions of traditional continuum mechanics. When MD atomistic models are used to generate powder diffraction patterns, strong deviations are observed from the usual paradigm of a small crystal perturbed by the strain field of lattice defects. The Krivoglaz-Wilkens model for dislocation effects of diffraction line profiles seems correct for the screw dislocation case if most parameters are known or strongly constrained. Nevertheless the practical implementation of the model, i.e., a free refinement of all microstructural parameters, leads to instability. Possible effects of the experimental practice based on Line Profile Analysis are discussed. read less

Abstract: Quasi-static (0.0033 s−1) and dynamic (103 s−1) compression experiments were performed on single crystal copper along ⟨100⟩ and ⟨110⟩ directions and best-fit parameters for the Johnson-Cook (JC) material model, which is an important input to hydrodynamic simulations for shock induced fracture, have been obtained. The deformation of single crystal copper along the ⟨110⟩ direction showed high yield strength, more strain hardening, and less strain rate sensitivity as compared to the ⟨100⟩ direction. Although the JC model at the macro-scale is easy to apply and describes a general response of material deformation, it lacks physical mechanisms that describe the influence of texture and initial orientation on the material response. Hence, a crystal plasticity model based on the theory of thermally activated motion of dislocations was used at the meso-scale, in which the evolution equations permit one to study and quantify the influence of initial orientation on the material response. Hardening parameters of the... read less

Abstract: Uniform-acceptance force-bias Monte Carlo (fbMC) methods have been shown to be a powerful technique to access longer timescales in atomistic simulations allowing, for example, phase transitions and growth. Recently, a new fbMC method, the time-stamped force-bias Monte Carlo (tfMC) method, was derived with inclusion of an estimated effective timescale; this timescale, however, does not seem able to explain some of the successes the method. In this contribution, we therefore explicitly quantify the effective timescale tfMC is able to access for a variety of systems, namely a simple single-particle, one-dimensional model system, the Lennard-Jones liquid, an adatom on the Cu(100) surface, a silicon crystal with point defects and a highly defected graphene sheet, in order to gain new insights into the mechanisms by which tfMC operates. It is found that considerable boosts, up to three orders of magnitude compared to molecular dynamics, can be achieved for solid state systems by lowering of the apparent activation barrier of occurring processes, while not requiring any system-specific input or modifications of the method. We furthermore address the pitfalls of using the method as a replacement or complement of molecular dynamics simulations, its ability to explicitly describe correct dynamics and reaction mechanisms, and the association of timescales to MC simulations in general. read less

Abstract: Molecular dynamics approach is used to simulate hydrogen (H) diffusion in zirconium. Zirconium alloys are used in fuel channels of many nuclear reactors. Previously developed embedded atom method (EAM) and modified embedded atom method (MEAM) are tested and a good agreement with experimental data for lattice parameters, cohesive energy, and mechanical properties is obtained. Both EAM and MEAM are used to calculate hydrogen diffusion in zirconium. At higher temperatures and in the presence of hydrogen, MEAM calculation predicts an unstable zirconium structure and low diffusion coefficients. Mean square displacement (MSD) of hydrogen in bulk zirconium is calculated at a temperature range of 500–1200 K with diffusion coefficient at 500 K equals 1.92 10−7 cm2/sec and at 1200 K has a value 1.47 10−4 cm2/sec. Activation energy of hydrogen diffusion calculated using Arrhenius plot was found to be 11.3 kcal/mol which is in agreement with published experimental results. Hydrogen diffusion is the highest along basal planes of hexagonal close packed zirconium. read less

Abstract: A new method, the Extended Temperature-Accelerated Dynamics (XTAD), is introduced for modeling long-timescale evolution of large rare-event systems. The method is based on the Temperature-Accelerated Dynamics approach [M. Sørensen and A. Voter, J. Chem. Phys. 112, 9599 (2000)], but uses full-scale parallel molecular dynamics simulations to probe a potential energy surface of an entire system, combined with the adaptive on-the-fly system decomposition for analyzing the energetics of rare events. The method removes limitations on a feasible system size and enables to handle simultaneous diffusion events, including both large-scale concerted and local transitions. Due to the intrinsically parallel algorithm, XTAD not only allows studies of various diffusion mechanisms in solid state physics, but also opens the avenue for atomistic simulations of a range of technologically relevant processes in material science, such as thin film growth on nano- and microstructured surfaces. read less

Abstract: We investigate the effect of size on intrinsic dissipation in nano-structures. We use molecular dynamics simulation and study dissipation under two different modes of deformation: stretching and bending mode. In the case of stretching deformation (with uniform strain field), dissipation takes place due to Akhiezer mechanism. For bending deformation, in addition to the Akhiezer mechanism, the spatial temperature gradient also plays a role in the process of entropy generation. Interestingly, we find that the bending modes have a higher Q factor in comparison with the stretching deformation (under the same frequency of operation). Furthermore, with the decrease in size, the difference in Q factor between the bending and stretching deformation becomes more pronounced. The lower dissipation for the case of bending deformation is explained to be due to the surface scattering of phonons. A simple model, for phonon dynamics under an oscillating strain field, is considered to explain the observed variation in diss... read less

Abstract: We use density functional theory and the atomistic Green's function method (AGF) to study the effect of bonding on phonon transmission and thermal boundary conductance (TBC) at the interface of metals (Au, Cu, and Ti) and single layer graphene (SLG)/multi-layer graphene (MLG). Our analysis shows that the TBC across Ti/SLG/Ti interfaces (∼500 MW m−2 K−1) is significantly larger than the TBC across Cu/SLG/Cu (∼10 MW m−2 K−1) and Au/SLG/Au (∼7 MW m−2 K−1) interfaces. However, the TBC across Ti/MLG/Ti (∼40 MW m−2 K−1) is an order of magnitude lower compared to TBC at the Ti/SLG/Ti interface, whereas the TBC at Cu/MLG/Cu and Au/MLG/Au interfaces are similar to those of Cu/SLG/Cu and Au/SLG/Au, respectively. We find that this substantial decrease in TBC at the Ti/MLG/Ti interface is a result of phonon mismatch between the graphene layer bonded to Ti and the non-bonded graphene layers. The effect of number of graphene layers on TBC at Cu/MLG/Cu and Au/MLG/Au interfaces is relatively insignificant because of the weak interactions at these metal/graphene interfaces. It was observed that the moderate attenuation of Ti/C bonding strength can enhance the phonon coupling between the graphene layers bonded to Ti and non-bonded graphene layers, and can increase the TBC across Ti/MLG/Ti by ∼100%. This impact of interfacial bonding strength on TBC at metal/MLG interfaces, predicted by AGF calculations, is further confirmed by non-equilibrium molecular dynamics simulations which show the transition of thermal transport mechanism from metal/graphene dominated resistance to graphene/graphene dominated resistance as the metal/graphene bonding strength increases in the metal/MLG/metal structure. read less

Abstract: The growth process of carbon nanotube (CNT)–graphene 3D junctions on copper templates with nano-holes was simulated with classical molecular dynamic (MD) simulation. The CNT, graphene and their seamlessly C–C bonded junction can form simultaneously on the templates without catalysts. There are two mechanisms of junction formation: (i) CNT growth over the holes that are smaller than 3 nm, and (ii) CNT growth inside the holes that are larger than 3 nm. The tensile strengths of the as-grown C–C junctions, as well as the junctions embedded with metal nanoparticles (catalysts), were determined by a quantum mechanics MD simulation method. Metal nanoparticles as catalysts remaining in the junctions significantly reduce the fracture strength and fracture energy, making them brittle and weak. Among the junctions, the seamlessly C–C bonded junctions show the highest tensile strength and fracture energy due to their unique structure. This work provides a theoretical basis and route for synthesizing high-quality single-layer CNT–graphene nanostructures. read less

Abstract: To reveal the fracture failure mechanisms of single crystals at elevated temperatures, a new temperature-dependent ideal tensile strength model for solids has been developed, based on the critical strain principle. At the same time, the uniaxial tensile strength model, based on the critical failure energy density principle for isotropic materials that was presented in the previous study, is generalized to multi-axial loading and to cubic single crystals. The relationship between the two models is discussed, and how to obtain the material properties needed in the calculations is summarized. The two well-established models are used to predict the temperature-dependent ideal tensile strength of W, Fe and Al single crystals. The predictions from the critical strain principle agree well with the predictions from the critical failure energy density principle. The theoretical values from the critical strain principle at 0 K is in reasonable agreement with the ab initio results. The study shows that the temperature dependence of the ideal tensile strength is similar to that of Young’s modulus; that is, the ideal tensile strength firstly remains approximately constant and then decreases linearly with the temperature. The fracture failure for single crystals at elevated temperatures has been identified, for the first time, as a strain-controlled criterion. read less

Abstract: We use molecular dynamics method to study the strengthening effect of graphene-metal nanolayered composites under shock loading. The graphene interfaces have the advantages of both strong and weak interfacial features simultaneously, which solves a strengthening paradox of interfacial structures. On one hand, the weak bending stiffness of graphene leads to interlayer reflections and weakening the shock wave. On the other hand, the strong in-plane sp2-bonded structures constrain the dislocations and heal the material. The elastic recovery due to graphene interfacial constraints plays an important role in the strengthening effect, and the shock strength can be enhanced by decreasing the interlayer distance. This interface with strong/weak duality should lead to an improved fundamental understanding on the dynamic mechanism of composites with interfacial structures. read less

Abstract: The distinct characteristics of short pulse laser interactions with a metal target under conditions of spatial confinement by a solid transparent overlayer are investigated in a series of atomistic simulations. The simulations are performed with a computational model combining classical molecular dynamics (MD) technique with a continuum description of the laser excitation, electron-phonon equilibration, and electronic heat transfer based on two-temperature model (TTM). Two methods for incorporation of the description of a transparent overlayer into the TTM-MD model are designed and parameterized for Ag-silica system. The material response to the laser energy deposition is studied for a range of laser fluences that, in the absence of the transparent overlayer, covers the regimes of melting and resolidification, photomechanical spallation, and phase explosion of the overheated surface region. In contrast to the irradiation in vacuum, the spatial confinement by the overlayer facilitates generation of sustain... read less

Abstract: We propose a strategy to obtain a hyperelastic constitutive law for film-like systems from molecular dynamics (MD) simulations. The aim is to furnish a computationally efficient continuum model with this description of the material. In particular, two different methods are suggested, both of which consist of virtual experiments that are performed on the material to sample systematically the stress–strain relation. The latter is then fitted to a suitable functional form. We use a polymeric self-assembled monolayer, which spans a height of only a few nanometers, as a test case. Having determined the response function, we then apply it within a finite-element simulation of a continuum mechanical nanoindentation problem. Several contact quantities such as normal reaction forces and the contact geometry are extracted from these calculations and are compared to those from an analogous, fully atomistic nanoindentation simulation. We find that the considered benchmark quantities as obtained from the continuum surrogate model reproduce well the corresponding values of the MD simulation. read less

Abstract: In the present work we elucidate the deformation mechanisms of twinned Au nanowires (NWs) under three-point bending by means of molecular dynamics simulations. We further investigate the effects of twin boundary orientation, NW diameter and twin boundary spacing on the mechanical properties and deformation behaviors of NWs. Our simulation results reveal that dislocation slip, twin boundary associated mechanisms and deformation twinning work in parallel in the heterogeneous localized plastic deformation of twinned Au NWs under bending. The dislocation–twin boundary interactions can be significantly altered by changing the twin boundary orientation as well as the bending direction. Furthermore, the NW diameter and twin boundary spacing strongly influence the competition between individual deformation mechanisms, which in turn leads to extrinsic and intrinsic size effects on the strength and the bending ductility of the NWs. read less

Abstract: We present investigations of pull-out tests on CNTs embedded in palladium by means of molecular dynamics (MD) and compare our results of maximum pull-out forces with values of nano scale in situ pull-out tests inside a scanning electron microscope (SEM). Our MD model allows the investigation of crucial influencing parameters on the interface behaviour, like CNT diameter, intrinsic CNT defects and functional groups. For the experiments we prepared simple specimens using silicon substrates and wafer level compliant technologies. We realised the nano scale experiment with a nanomanipulation system supporting an AFM cantilever with known stiffness as a force sensing element inside a SEM. Greyscale correlation has been used to evaluate the cantilever deflection. From simulations derived maximum pull-out forces are approximately 17 nN and depend on the existence of intrinsic defects or functional groups and weakly on temperature. Experimentally obtained maximum pull-out forces with values between 16-29 nN are in good agreement with the computational predictions. Our results are of significant interest for the design and a failure-mechanistic treatment of future mechanical sensors with integrated single-walled CNTs showing high piezoresistive gauge factor or other nano scale systems incorporating CNT-metal interfaces. read less

Abstract: The addition of the embedding energy term to pair interaction contribution has made the embedded atom method (EAM) potentials a simple and vastly superior alternative to popular classical pair potentials. EAM relies on the ansatz that the embedding energy is a function of a linear superposition of spherically averaged atomic electron densities. This ansatz is taken to be self-evident and inviolate. Using density functional theory (DFT) calculations of a model face-centered cubic (fcc) Cu system, we systematically investigate the validity of this foundational ansatz of EAM. We conclude that it (1) agrees well with DFT calculations along a path with changing coordination and symmetry, (2) captures the exponential decrease of the background electron density with respect to distance, (3) demonstrates transferability as seen by agreement of electron densities for other non-fcc structures with first nearest neighbor (NN) coordination ranging from 4 to 12 and (4) fails to explain the behavior of background electron density with respect to second NN distance and arrangements. This failure may be remedied by including a fraction of the second NN atomic electron density in the background electron density, including angular contributions to the density, or including electron density rearrangement. These insights likely make EAM approaches more broadly applicable, more predictive and perhaps unique, and in the process broadly impact atomistic modeling. A new EAM potential is presented that for the first time reproduces electron densities from DFT calculations as well as experimental properties of Cu in the potential fitting. read less

Abstract: Adsorption and activation of CO2 can be considered as the first steps of the reaction mechanism of methanol synthesis via CO2 hydrogenation on Cu-based bimetallic clusters. In this work, the adsorption properties of CO2 on Cu55, Cu54Ni1, and Cu42Ni13 clusters with highly symmetric cuboctaheral (Cubo), decahedral (Dec), and icosahedral (Ico) structures are studied by density functional theory calculations. It is found that icosahedral Cu42Ni13 cluster exhibits the strongest CO2 adsorption ability with all the −Coo, −Oco, and −OcO– adsorption models, compared to the icosahedral Cu55 and Cu54Ni1 clusters. In addition, the structural transformation from cuboctaheral and decahedral to icosahedral clusters upon CO2 adsorption is found, which is attributed to the fact that the stability of these clusters follows the order Cubo < Dec < Ico. Our results show that composition and geometric relaxation can modify the adsorption properties of CO2 on Cu–Ni bimetallic clusters, which can provide useful insights for the ... read less

Abstract: Swift heavy ion (SHI) irradiation of amorphous SiO2 that contains metal nanocrystals can be used to transform the shape of the particles into peculiar asymmetric ones not easily achievable by other means. Using a molecular dynamics simulation framework augmented to include the electronic excitations of the SHIs, we predict that the reshaping of spherical particles into nanorods occurs continuously during consecutive ion impacts by a dynamic crystal–liquid–crystal phase transition of metal particle with the flow of liquid phase into an underdense track core in silica. The simulated nanocrystals are shown to have a saturation width that agrees with experiments. read less

Abstract: We report the synthesis, structural characterization, and atomistic simulations of AgPd-Pt trimetallic (TM) nanoparticles. Two types of structure were synthesized using a relatively facile chemical method: multiply twinned core-shell, and hollow particles. The nanoparticles were small in size, with an average diameter of 11 nm and a narrow distribution, and their characterization by aberration corrected scanning transmission electron microscopy allowed us to probe the structure of the particles at an atomistic level. In some nanoparticles, the formation of a hollow structure was also observed, that facilitates the alloying of Ag and Pt in the shell region and the segregation of Ag atoms on the surface, affecting the catalytic activity and stability. We also investigated the growth mechanism of the nanoparticles using grand canonical Monte Carlo simulations, and we have found that Pt regions grow at overpotentials on the AgPd nanoalloys, forming 3D islands at the early stages of the deposition process. We found very good agreement between the simulated structures and those observed experimentally. read less

Abstract: Effect of the micro texture of electroplated copper thin film interconnections on stress-induced migration was investigated experimentally and theoretically. The micro texture of electroplated copper thin films changed drastically as a function of the annealing temperature after the electroplating. However, stress-induced migration was activated even though the thin film interconnection was kept at room temperature after annealing. As a result, voids and hillocks appeared on the thin film interconnection. This is because high residual stress was caused by shrinkage of the thin film interconnection due to the densification caused by recrystallization. Molecular dynamics simulations showed that the diffusivity of copper atoms along grain boundaries with low crystallinity was enhanced significantly by high tensile residual stress. Therefore, the grain boundary diffusion accelerated by tensile residual stress is the main reason for the formation of hillocks and voids in the thin film interconnection after annealing. read less

Abstract: The peeling process and average peeling force of a graphene (GE) sheet on a corrugated surface are investigated using molecular dynamics simulation. It is found that the peeling behaviour varies with the substrate surface roughness and the peeling angle. Three kinds of typically peeling behaviours include (a) GE sheet directly passing the valley of the substrate roughness; (b) bouncing off from the substrate; and (c) continuously peeling off similarly to that on a flat substrate. As a result, the average peeling force is strongly dependent of the peeling behaviours. Furthermore, some interesting phenomena are caught, such as partial detaching and partial sliding of GE sheet in the valley of the substrate roughness, which are mainly due to the effects of pre-tension in GE sheet and the reduction of friction resistance. The results in this paper should be useful for the design of nano-film/substrate systems. read less

Abstract: Twinning in hexagonal close-packed Mg alloys has been reported to be unfavorable when the grain size is reduced below a couple of microns and suppressed at the nanoscale. Using high-resolution transmission electron microscopy, we present evidence of nanotwins (<1 nm) in nanocrystalline Mg–Al alloys processed by cryomilling. The commonly observed twinning modes for coarse-grained Mg are identified and supported with atomistic molecular dynamics simulations. The specific thermomechanical conditions offered by cryomilling facilitate the generation of deformation twins that are not observed with conventional deformation processing methods. read less

Abstract: Room temperature nanojoining is an important phenomenon that has to be understood well for use in different applications, for example, for assembly of nanoscale building blocks into nanoscale and microscale structures and devices. However, the mechanism for nanoparticle joining at room temperature is not well established. In this research, we employed molecular dynamics simulation to explain how and why silver nanodisks are joined/assembled but with their original shape unchanged. To support our theoretical observations, we compared our simulation results to SEM and HRTEM observations of joined silver nanodisks. It was found that joining at a wide temperature range (1–500 K) can be done through short movement and rearrangement of surface atoms and subsequent elastic or plastic deformation of the particles, resulting in perfect crystal alignment at the joint interface. Our simulation shows the crystal defects such as dislocations due to initial lattice mismatch of the crystals can be sintered out to yield ... read less

Abstract: We present a computational, atomistic electrodynamics investigation of the effects of planar defects on the optical properties of silver nanocubes, where the planar defects we considered are different surface orientations, twins, partial dislocations, and full dislocations. We find that for nanocubes smaller than about 3 nm, the optical response is very sensitive to the specific surface structure resulting from the defects. However, the sensitivity, as measured by shifts in the plasmon resonance wavelength, is strongly reduced at larger sizes because of the decreasing importance of surface effects even when the majority of the atomic deformation due to the crystal defects is contained within the interior of the nanocube. Overall, this study suggests that the effects of individual crystalline defects on the optical properties of nanostructures can be safely ignored for nanostructure sizes larger than about 5 nm. read less

Abstract: The effect of cooling rate on the microstructure evolution of liquid Cu nanoparticles during their solidification process is investigated by using a molecular dynamics simulation based on the embedded atom method (EAM) potential developed by Foiles et al.. The potential energy analysis, the pair distribution function and the common neighbor analysis have been used. The results show that the solidification point increases with decreasing cooling rate and that the solidification of the microstructure of Cu nanoparticles varies with the cooling rate. The microstructure consists of fcc, hcp and bcc crystals or mixtures, though the fcc structure dominates, except in the amorphous state. An amorphous structure was obtained when the cooling rate reached 1.0 × 1013 K/s or higher while crystallization degree increased with decreasing cooling rate, and the total content of crystal structures reached to 95% when the cooling rate dropped to 4.0 × 1011 K/s, which was nearly a perfect crystal structure. The results also indicate that a single-crystal nanoparticle will not be obtained by quenching the liquid metal under various cooling rates. read less

Abstract: An ensemble averaging approach was investigated for its accuracy and convergence against time averaging in computing continuum quantities such as stress, heat flux and temperature from atomistic scale quantities. For this purpose, ensemble averaging and time averaging were applied to evaluate Hardy's thermomechanical expressions (Hardy 1982 J. Chem. Phys. 76 622–8) in equilibrium conditions at two different temperatures as well as a nonequilibrium process due to shock impact on a Ni crystal modeled using molecular dynamics simulations. It was found that under equilibrium conditions, time averaging requires selection of a time interval larger than the critical time interval to obtain convergence, where the critical time interval can be estimated using the elastic properties of the material. The reason for this is because of the significant correlations among the computed thermomechanical quantities at different time instants employed in computing their time average. On the other hand, the computed thermomechanical quantities from different realizations in ensemble averaging are statistically independent, and thus convergence is always guaranteed. The computed stress, heat flux and temperature show noticeable difference in their convergence behavior while their confidence intervals increase with temperature. Contrary to equilibrium settings, time averaging is not equivalent to ensemble averaging in the case of shock wave propagation. Time averaging was shown to have poor performance in computing various thermomechanical fields by either oversmoothing the fields or failing to remove noise. read less

Abstract: The role of disclinations in the processing, microstructure and properties of bulk nanostructured materials is reviewed. Models of grain subdivision during severe plastic deformation (SPD) based on the disclination concept, a structural model of the bulk nanostructured materials processed by SPD are presented. The critical strength of triple junction disclinations is estimated. Kinetics of relaxation of triple junction disclinations and their role in the grain boundary diffusion are studied. read less

Abstract: Sintering of Ag nanoparticles (NPs) is increasingly being used as a driving mechanism for joining in the microelectronics industry. We therefore performed molecular dynamics simulations based on the embedded atom method (EAM) to study pressureless sintering kinetics of two Ag NPs in the size range of (4 to 20 nm), and sintering of three and four Ag NPs of 4 nm diameter. We found that the sintering process passed through three main stages. The first was the neck formation followed by a rapid increase of the neck radius at 50K for 20 nm particles and at 10K for smaller NPs. The second was characterized by a gradual linear increase of the neck radius to particle radius ratio as the temperature of the sintered structure was increased to the surface premelting point. Different than previous sintering studies, a twin boundary was formed during the second stage that relaxed the sintered structure and decreased the average potential energy (PE). The third stage of sintering was a rapid shrinkage during surface premelting of the sintered structure. Based on pore geometry, densification occurred during the first stage for three 4 nm particles and during the second stage for four 4 nm particles. Sintering rates obtained by our simulation were higher than those obtained by theoretical models generally used for predicting sintering rates of microparticles. [doi:10.2320/matertrans.MD201225] read less

Abstract: CNT/metal interfaces under mechanical loads are investigated using molecular dynamics by simulating pull-out tests of single walled carbon nanotubes (CNTs) emdedded in single crystal gold lattices. As a result of our simulations we present obtained force-displacement data. We investigated the influence of two different Lennard Jones (LJ) coefficients pairs, two CNT types and three lattice directions of the gold matrix with respect to the embedding direction. Additionally we incorporated structural defects into our model and report on their influence. The change of the CNT type leads to a change in the maximum pull-out force. Here we attribute this to the change in CNT diameter, where a bigger diameter entails an increased maximum pull-out force. Changing the LJ coefficient pair has a strong impact on the maximum pull-out forces, where a higher bonding energy results in a higher maximum pull-out force. Defects also show a positive effect on the maximum pull-out force. The presented results have impact on bonding strength of CNT/metal interfaces. read less

Abstract: Despite the fundamental nature and practical importance of melting, there is still no generally accepted theory of this ubiquitous phenomenon. Even the earliest simulations of melting of hard discs by Alder and Wainwright indicated the active role of collective atomic motion in melting and here we utilize molecular dynamics simulation to determine whether these correlated motions are similar to those found in recent studies of glass-forming (GF) liquids and other condensed, strongly interacting, particle systems. We indeed find string-like collective atomic motion in our simulations of "superheated" Ni crystals, but other observations indicate significant differences from GF liquids. For example, we observe neither stretched exponential structural relaxation, nor any decoupling phenomenon, while we do find a boson peak, findings that have strong implications for understanding the physical origin of these universal properties of GF liquids. Our simulations also provide a novel view of "homogeneous" melting in which a small concentration of interstitial defects exerts a powerful effect on the crystal stability through their initiation and propagation of collective atomic motion. These relatively rare point defects are found to propagate down the strings like solitons, driving the collective motion. Crystal integrity remains preserved when the permutational atomic motions take the form of ring-like atomic exchanges, but a topological transition occurs at higher temperatures where the rings open to form linear chains similar in geometrical form and length distribution to the strings of GF liquids. The local symmetry breaking effect of the open strings apparently destabilizes the local lattice structure and precipitates crystal melting. The crystal defects are thus not static entities under dynamic conditions, such as elevated temperatures or material loading, but rather are active agents exhibiting a rich nonlinear dynamics that is not addressed in conventional "static" defect melting models. read less

Abstract: In this work we apply a Lagrangian kernel-based estimator of continuum fields to atomic data to estimate the J-integral for the emission dislocations from a crack tip. Face-centered cubic (fcc) gold and body-centered cubic (bcc) iron modeled with embedded atom method (EAM) potentials are used as example systems. The results of a single crack with a K-loading compare well to an analytical solution from anisotropic linear elastic fracture mechanics. We also discovered that in the post-emission of dislocations from the crack tip there is a loop size-dependent contribution to the J-integral. For a system with a finite width crack loaded in simple tension, the finite size effects for the systems that were feasible to compute prevented precise agreement with theory. However, our results indicate that there is a trend towards convergence. read less

Abstract: Phonon dynamics and phonon thermal conductivity of f.c.c. Cu are investigated in detail in the temperature range 200 1300 K within the framework of equilibrium molecular dynamics simulations making use of the Green-Kubo formalism and one of the most reliable embedded-atom method potentials. It is found that the temporal decay of the heat current autocorrelation function of the f.c.c. Cu model at low and intermediate temperatures demonstrates a more complex behaviour than the two-stage decay observed previously for the f.c.c. Ar model. After the first stage of decay, it demonstrates a peak in the temperature range 200 800 K. The intensity of the peak decreases as the temperature increases. At 900 K, it transforms to a shoulder which diminishes almost entirely at 1200 K. It is suggested that the peak may be activated by the influence of the Cauchy pressure in f.c.c. Cu on the phonon dynamics. A decomposition model of the heat current autocorrelation function of a monatomic f.c.c. lattice is introduced. This model can capture all contributions to the function discussed in the literature. It is found that the temperature dependence of the phonon thermal conductivity of the f.c.c. Cu model is in good agreement with previous calculations on the f.c.c. Ar model which follows an exponent close to-1.4, i.e. varies more rapidly than the T-1 law predicted by the theory. The calculated phonon thermal conductivity of the f.c.c. Cu is found to be about one order of magnitude higher than the f.c.c. Ar. This is explained by the inclusion of the electronic contribution to the bulk lattice properties during the fitting of the embedded-atom method potential functions to the experimental or ab initio data. It is demonstrated that the electronic contribution to the total thermal conductivity of f.c.c. Cu dominates over the whole studied temperature range. Nevertheless, the phonon contribution increases as the temperature decreases. The contribution can be estimated to be about 0.5 % at 1300 K and about 5 % at 200 K. read less

Abstract: To study the initial plasticity during the nanoindentation of face-centered cubic bicrystal materials at the micro- and nanoscales, a $$\sum {5(310)/[001]} {{\theta }} = 53.1\circ $$ symmetrical tilt grain boundary (GB) model of bicrystal copper was constructed using the quasicontinuum method. The nanoindentation process of the model was then simulated, and a group of indents across the GB during a bicrystal copper nanoindentation experiment were performed. The effect of the GB on the incipient yield was studied when the transition from elastic to plastic deformation and the first dislocation emission occur. The results show that the maximum incipient load appears in the center of the boundary; the load first increases and then gradually decreases until it presents no further significant changes when the indenter is far from the GB. It is observed that theoretical simulation results are in good agreement with those of the experimental measurement. The incipient yield force was affected by the size and the position of the indenter, the structure of the boundary, and the first dislocation emission. read less

Abstract: Nanoporous gold (NP-Au) exhibits microscale plasticity, but macroscopically fails in a relatively brittle manner. This current study suggests that a core-shell structure can increase both ductility and strength of NP-Au. A core Au foam structure was created using conventional dealloying methods with average ligament size of 60 nm. Nickel was then electroplated on to the NP-Au with layer thicknesses ranging from 2.5 nm to 25 nm. Nanoindentation demonstrated a significant increase in the hardness of the coated Np-Au, to about five times of that of the pure Np-Au, and a decrease in creep by increasing the thickness of the coated Ni layer. Molecular dynamics simulations of Au–Ni ligaments show the same trend of strengthening behavior with increasing Ni thickness suggesting that the strengthening mechanisms of the Np-Au are comparable to those for fcc nano ligaments. The simulations demonstrate two different strengthening mechanisms with the increased activity of the twins in plated Au–Ni ligaments, which leads to more ductile behavior, as opposing to the monolithic Au ligaments where nucleation of dislocations govern the plasticity during loading. read less

Abstract: Femtosecond laser irradiation of a gold nanorod has been simulated by a method that couples two-temperature model into molecular dynamics. Numerical results show that the surface premelting occurs prior to the initiation of planar defect and propagates from the surface layer into the inside of nanorod. Pressure relaxation leads to high-frequency temperature oscillation and two-way transformation between fcc and disordered atoms produced by the dynamic stresses. Partial dislocation cores are initiated on the crystal surfaces due to high stresses, and then noticeable planar defects including stacking faults and twin boundaries on {111} close-packed planes are developed. Finally, only parallel twin boundaries are present in the nanorod, showing favorable agreement with the experimental observation. read less

Abstract: Following a simple thermodynamic model, which predicts that an array of non-wettable pores can be filled by dewetting of sufficiently thin films, we use molecular dynamics to simulate the rupture of nanometre-thick liquid Au films on nanoporous substrates. Our simulations clearly exhibit spinodal dewetting and hole nucleation, and some of the metal is indeed absorbed by non-wettable pores solely as a virtue of the Laplace pressure acting on dewetted droplets and rivulet-like structures. Finally, we show that the fraction of absorbed Au can be increased through patterning of the initial film. read less

Abstract: Using molecular dynamics simulations, we demonstrate that the efficiency of heat exchange between a solid and a gas can be maximized by functionalizing solid surface with organic self-assembled monolayers (SAMs). We observe that for bare metal surfaces, the thermal accommodation coefficient (TAC) strongly depends on the solid-gas interaction strength. For metal surfaces modified with organic SAMs, the TAC is close to its theoretical maximum and is essentially independent from the SAM-gas interaction strength. The analysis of the simulation results indicates that softer and lighter SAMs, compared to the bare metal surfaces, are responsible for the greatly enhanced TAC. read less

Abstract: The concept of the directional pair distribution function is proposed for an atomistic level interpretation of the line profile broadening in powder diffraction patterns of nanocrystalline materials. read less

Abstract: Nanoporous metals are a class of novel nanomaterials with potential applications in many fields such as sensing, catalysis, and fuel cells. The present paper is aimed to investigate atomic mechanisms associated with the uniaxial tensile deformation behavior of nanoporous gold. A phase field method is adopted to generate the bicontinuous open-cell porous microstructure of the material. Molecular dynamics simulations then reveal that the uniaxial tensile deformation in such porous materials is accompanied by an accumulation of stacking faults in ligaments along the loading direction and their junctions with neighboring ligaments, as well as the formation of Lomer–Cottrell locks at such junctions. The tensile strain leads to progressive necking and rupture of some ligaments, ultimately resulting in failure of the material. The simulation results also suggest scaling laws for the effective Young's modulus, yield stress, and ultimate strength as functions of the relative mass density and average ligament size ... read less

Abstract: An objective quasicontinuum (OQC) method is developed for simulating rodlike systems that can be represented as a combination of locally objective structures. An objective structure (OS) is one for which a group of atoms, called a “fundamental domain” (FD), is repeated using specific rules of translation and rotation to build a more complex structure. An objective Cauchy-Born rule defines the kinematics of the OS atoms in terms of a set of symmetry parameters and the positions of the FD atoms. The computational advantage lies in the capability of representing a large system of atoms through a small set of symmetry parameters and FD atom positions. As an illustrative example, we consider the deformation of a copper single-crystal nanobeam which can be described as an OS. OQC simulations are performed for uniform and nonuniform bending for two different orientations (nanobeam axis oriented along [111] and [100]) and compared with elastica results. In the uniform bending case, the [111]-oriented single-crystal nanobeam experiences elongation, while the [100]-oriented nanobeam experiences contraction in total length. The nonuniform bending allows for stretching, contraction, and bending as deformation. Under certain loading conditions, dislocation nucleation is observed within the FD. read less

Abstract: The understanding of deformation mechanisms in nanocrystalline metals is still not clear. At present, the role of grain boundary is gaining importance. Most of the studies are carried out through simulation methods. Nanocrystalline materials are normally porous in nature. In this work, the effect of porosity on the deformation behaviour (under compression) of nanocrystalline FCC metal (Pd) is investigated by molecular dynamics simulation. The results show that modulus of elasticity and flow stress of the nanocrystalline FCC metals decreases with increase in porosity. The atoms in grains adjacent to the pores are observed to move towards the void space. For a relaxed nanocrystalline metal, the grain boundary atoms are under compressive stress. read less

Abstract: Dual-mode vibration of nanowires (NWs) has been reported experimentally through actuation of the NW at its resonance frequency, which is expected to open up a variety of new modalities for nanoelectromechanical systems that could operate in the nonlinear regime. In this work, we utilize large-scale molecular dynamics simulations to investigate the dual-mode vibration of 〈1 1 0〉 Ag NWs with triangular, rhombic and truncated rhombic cross-sections. By incorporating the generalized Young–Laplace equation into the Euler–Bernoulli beam theory, the influence of surface effects on the dual-mode vibration is studied. Due to the different lattice spacings in the principal axes of inertia of the {1 1 0} atomic layers, the NW is also modelled as a discrete system to reveal the influence from such a specific atomic arrangement. It is found that the 〈1 1 0〉 Ag NW will be under a dual-mode vibration if the actuation direction deviates from the two principal axes of inertia. The predictions of the two first mode natural frequencies by the classical beam model appear underestimated compared with the MD results, which are found to be enhanced by the discrete model. Particularly, the predictions by the beam theory with the contribution of surface effects are uniformly larger than the classical beam model, which exhibit better agreement with MD results for a larger cross-sectional size. However, for ultrathin NWs, current consideration of surface effects still experiences certain inaccuracy. In all, for all different cross-sections, the inclusion of surface effects is found to reduce the difference between the two first mode natural frequencies. This trend is observed to be consistent with MD results. This study provides a first comprehensive investigation on the dual-mode vibration of 〈1 1 0〉 oriented Ag NWs, which is supposed to benefit the applications of NWs that act as a resonating beam. read less

Abstract: Large-scale molecular dynamics simulations are performed to characterize the effects of pre-existing surface defects on the vibrational properties of Ag nanowires. It is found that the first order natural frequency of the nanowire appears insensitive to different surface defects, indicating a defect insensitivity property of the nanowire's Young's modulus. In the meanwhile, an increase of the quality (Q)-factor is observed due to the presence of defects. Particular, a beat phenomenon is observed for the nanowire with the presence of a surface edge defect, which is driven by a single actuation. It is concluded that different surface defects could act as an effective mean to tune the vibrational properties of nanowires. This study sheds lights on the better understanding of nanowire's mechanical performance when surface defects are presented, which would benefit the development of nanowire-based devices. read less

Abstract: The use of a weakly binding ligand to facilitate sintering between particles and a planar substrate in the absence of pressure and at low homologous temperature has been explored. Ag nanoparticles in the 5-15 nm range suspended in water and stabilized by a BH4 complex were dropped onto a polished Ag substrate heated to 333 K. The Ag particles sintered to each other and to the substrate to form a largely pore free system. A molecular dynamics simulation is used to understand the theoretical limits of pressure free sintering and practical implications for adhesive systems based on Ag nanoparticle suspensions are discussed. read less

Abstract: Nanostructured metallic material (NMM) composites are a new class of materials that exhibit high structural stability, mechanical strength, high ductility, toughness and resistance to fracture and fatigue; these properties suggest that these materials can play a leading role in the future micromechanical devices. However, before those materials are put into service in any significant applications, many important fundamental issues remain to be understood. Among them, is the question of the strengthening of NMM using second phase particles and if the addition of precipitates will strengthen the structures in the same manner as in bulk crystalline solids. This issue is addressed in this work by performing molecular dynamics simulations on NMM with precipitates of various sizes and comparing the results with the same structure without precipitates. In this view, Cu/Nb bilayer thin films with spherical Nb particles inside the Cu layer were examined using molecular dynamics simulations and show a significant improvement on their mechanical behavior, compared to similar structures without particles. Furthermore, an analytical model is developed that explains the strengthening behavior of an NMM that has precipitates inside one layer. The theoretical results show a qualitative agreement with the finding of the atomistic simulations. read less

Abstract: Here, we uncover three new nanoplasticity mechanisms, operating in highly stressed interstitial-rich regions in face-centred-cubic (FCC) metals, which are particularly important in understanding evolution of surface stress and morphology of a FCC metal under low-energy noble-gas ion bombardments. The first mechanism is the configurational motion of self-interstitials in subsonic scattering during ion bombardments. We have derived a stability criterion of self-interstitial scattering during ion embedding, which consistently predicts the possibility of vacancy- and interstitial-rich double-layer formation for various ion bombardments. The second mechanism is the growth by gliding of prismatic dislocation loops (PDLs) in a highly stressed interstitial-rich zone. This mechanism allows certain prismatic dislocations with their Burgers vectors parallel to the surface to grow in subway-glide mode (SGM) during ion bombardment. The SGM growth creates a large population of nanometre-sized prismatic dislocations beneath the surface. The third mechanism is the Burgers vector switching of a PDL that leads to unstable eruption of adatom islands during certain ion bombardments of FCC metals. We have also derived the driving force and kinetics for the growth by gliding of prismatic dislocations in an interstitial-rich environment as well as the criterion for Burgers vector switching, which consistently clarifies previously unexplainable experimental observations. read less

Abstract: Using molecular dynamics simulations with the embedded atom method we calculate the core structure of a superdislocations in prism and basal planes and 2c + a superdislocations in pyramid planes. An analysis of the structure of the cores showed that the core is planar in the prismatic plane and nonplanar for screw superpartial dislocations in the basal plane. It is shown that the glissile 2c + a superdislocations have higher energy than the configurations of dislocation barriers. The influence of the core structure of superdislocations on the orientation dependence of the deformation behavior of Ti3Al is discussed. read less

Abstract: Internal stress in polycrystalline materials is an intrinsic attribute of the microstructure that affects a broad range of material properties. It is usually acquired through experiment in conjunction with continuum mechanics modelling, but its determination at nanometre and submicron scales is extremely difficult. Here, we report a bottom-up approach using atomistic calculation. We obtain the internal stress in polycrystalline copper with nanosized grains by first computing the stress associated with each atom and then sorting the stress into those associated with different self-equilibrating length scales, i.e. sample scale and grain cell, which gives type I, II and III residual stresses, respectively. The result shows highly non-uniform internal stress distribution; the internal stress depends sensitively on grain size and the grain shape anisotropy. Statistical distributions of the internal stresses, along with the means and variance, are calculated as a function of the mean grain size and temperature. The implementation of this work in assisting the interpretation of experimental results and predicting material properties is discussed. read less

Abstract: Hypervelocity impact properties of two different graphene armor systems are investigated using molecular dynamics simulations. One system is the so-called spaced armor which consists of a number of graphene plates spaced certain distance apart. Its response under normal impact of a spherical projectile is studied, focusing on the effect of the number of graphene monolayers per plate (denoted by n ) on the penetration resistance of the armor. We find that under normal impact by a spherical projectile the penetration resistance increases with decreasing number of monolayers per plate ( n ), and the best penetration resistance is achieved in the system with one graphene layer for each plate. Note that the monolayers in all the simulated multilayer graphene plates were AB-stacked. The second system being studied is the laminated copper/graphene composites with the graphene layers inside copper, on impact or back surface, or on both the impact and back surfaces. The simulation results show that under normal impact by a spherical projectile the laminated copper/graphene composite has much higher penetration resistance than the monolithic copper plate. The best efficiency is achieved when the graphene layers are on both the impact and back surfaces. read less

Abstract: Using molecular dynamics simulation, we have studied the low-energy sputtering at the energies near the sputtering threshold. Different projectile-target combinations of noble metal atoms (Cu, Ag, Au, Ni, Pd, and Pt) are simulated in the range of incident energy from 0.1 to 200 eV. It is found that the threshold energies for sputtering are different for the cases of M1 < M2 and M1 ≥ M2, where M1 and M2 are atomic mass of projectile and target atoms, respectively. The sputtering yields are found to have a linear dependence on the reduced incident energy, but the dependence behaviors are different for the both cases. The two new formulas are suggested to describe the energy dependences of the both cases by fitting the simulation results with the determined threshold energies. With the study on the energy dependences of sticking probabilities and traces of the projectiles and recoils, we propose two different mechanisms to describe the sputtering behavior of low-energy atoms near the threshold energy for the cases of M1 < M2 and M1 ≥ M2, respectively.Using molecular dynamics simulation, we have studied the low-energy sputtering at the energies near the sputtering threshold. Different projectile-target combinations of noble metal atoms (Cu, Ag, Au, Ni, Pd, and Pt) are simulated in the range of incident energy from 0.1 to 200 eV. It is found that the threshold energies for sputtering are different for the cases of M1 < M2 and M1 ≥ M2, where M1 and M2 are atomic mass of projectile and target atoms, respectively. The sputtering yields are found to have a linear dependence on the reduced incident energy, but the dependence behaviors are different for the both cases. The two new formulas are suggested to describe the energy dependences of the both cases by fitting the simulation results with the determined threshold energies. With the study on the energy dependences of sticking probabilities and traces of the projectiles and recoils, we propose two different mechanisms to describe the sputtering behavior of low-energy atoms near the threshold energy for the... read less

Abstract: This work presents key modeling aspects that are central to the manipulation of the decoration of metallic nanoparticles by a thin shell of a metal of different chemical nature. The concept of underpotential deposition is generalized to nanoparticles. An all-atom model, taking into account many-body interactions by means of the embedded atom potential, was used to represent nanoparticles of different sizes and atomic adsorbates on them. A full set of state-of-the-art computer simulations are performed for a model system, showing that selective decoration of facets is possible. The trends observed in the present work are in good qualitative agreement with experimental data reported very recently. read less

Abstract: Ultrafast laser-induced melting in a gold thin film is simulated by an integrated continuum-atomistic method with the extended Drude model for dynamic optical properties. The local order parameter of atoms is used to identify solid and liquid regions. It is shown that the film is superheated in the early nonequilibrium stage and the melted region grows very quickly with a very high rate of melting up to ∼13,300 m/s. It is also found that the continuum approach could significantly underestimate the ultrafast phase-change response, and temperature-dependent optical properties should be considered in atomic-level modeling for ultrafast laser heating. read less

Abstract: Molecular dynamics simulations are used to study the competitive freezing of icosahedral, decahedral, polydecahedral, and face-centered cubic structures in gold nanoclusters for a range of cluster sizes and temperatures. Measuring the probability of observing each cluster type in an ensemble of freezing events, along with the overall rate at which liquid drops freeze to any structure, allows us to calculate the rate of formation for each structure. An analysis of the freezing trajectories of the clusters reveals that both the icosahedron and the decahedron structures nucleate through the initial formation of a 5-fold symmetric cap. The icosahedron then continues to grow through the sequential growth of neighboring tetrahedral subunits, while the decahedral cluster grows out from the cap along the central 5-fold axis of symmetry. read less

Abstract: The need for modeling and simulating multiscale structural responses to extreme loading conditions has brought about the challenging tasks of bridging different spatial and temporal scales within a unified framework. Based on the available experimental and computational capabilities, a simple approach has been proposed to formulate a hyper-surface in both spatial and temporal domains to predict combined specimen size and loading rate effects on the material properties. A systematic investigation has been performed over the last several years to understand the combined size, rate and thermal effects on the properties and deformation patterns of representative materials with different nanostructures and under various types of loading conditions. In this presentation, recent findings are presented about combined size and rate effects on the material properties of single crystal copper nanobeams under impact loading, with a focus on the link between the inverse Hall-Petch phenomenon and classical Hall-Petch phenomenon. It appears from the preliminary results that the inverse Hall-Petch behavior in single crystal materials is mainly due to the formation and evolution of disordered atoms as compared with the physics behind the inverse Hall-Petch behavior in nanocrystalline materials. read less

Abstract: Based on the molecular dynamics (MD) simulation and the classical Euler-Bernoulli beam theory, a fundamental study of the vibrational performance of the Ag nanowire (NW) is carried out. A comprehensive analysis of the quality (Q)-factor, natural frequency, beat vibration, as well as high vibration mode is presented. Two excitation approaches, i.e., velocity excitation and displacement excitation, have been successfully implemented to achieve the vibration of NWs. Upon these two kinds of excitations, consistent results are obtained, i.e., the increase of the initial excitation amplitude will lead to a decrease to the Q-factor, and moderate plastic deformation could increase the first natural frequency. Meanwhile, the beat vibration driven by a single relatively large excitation or two uniform excitations in both two lateral directions is observed. It is concluded that the nonlinear changing trend of external energy magnitude does not necessarily mean a non-constant Q-factor. In particular, the first order ... read less

Abstract: Molecular dynamics simulations are performed to study size effects on the impact response of copper nanobeam targets subjected to impacts by copper nanobeam flyers with different impact velocities. It is found that the Hugoniot response is size-dependent, while the aspect ratio – that is, the ratio of flyer and target nanobeam heights – has a small effect. It is also observed that the propagation speed of a disordering front generated at the impact surface is close to the shock wave speed initially, but decreases as dislocations form. The thermal gradient in the target is mainly due to the quasi-temperature difference (transient spatial localization of kinetic energy) between hexagonal-close-packed atoms and face-centered-cubic atoms. The findings for the impact stress, defect evolution, and quasi-temperature could be useful for better understanding the responses of nanosystems to extreme loading conditions. read less

Abstract: Monte Carlo (MC) methods have a long-standing history as partners of molecular dynamics (MD) to simulate the evolution of materials at the atomic scale. Among these techniques, the uniform-acceptance force-bias Monte Carlo (UFMC) method [ G. Dereli Mol. Simul. 8 351 (1992)] has recently attracted attention [ M. Timonova et al. Phys. Rev. B 81 144107 (2010)] thanks to its apparent capacity of being able to simulate physical processes in a reduced number of iterations compared to classical MD methods. The origin of this efficiency remains, however, unclear. In this work we derive a UFMC method starting from basic thermodynamic principles, which leads to an intuitive and unambiguous formalism. The approach includes a statistically relevant time step per Monte Carlo iteration, showing a significant speed-up compared to MD simulations. This time-stamped force-bias Monte Carlo (tfMC) formalism is tested on both simple one-dimensional and three-dimensional systems. Both test-cases give excellent results in agreement with analytical solutions and literature reports. The inclusion of a time scale, the simplicity of the method, and the enhancement of the time step compared to classical MD methods make this method very appealing for studying the dynamics of many-particle systems. read less

Abstract: The most interesting applications of nanotubes include their use as storage media for atoms and small molecules, as nanoscale capsules for chemical reactions, and as nanopipettes for material delivery. The geometrical transformation of metallic copper nanowires, confined in graphitic coating, into crystalline nanoparticles of up to tenfold increased diameter is reported. In situ transmission electron microscopy images at 500 °C, recorded as movies, provide an exceptional real‐time visualization of Cu draining out of the carbon coating. The solid content of the carbon tube is effectively evacuated over micrometer distances towards the open end, transforming each nanowire into a single monocrystalline, facetted Cu particle. Kinetic Monte Carlo simulations propose that this dramatic morphological transformation is driven by surface diffusion of Cu atoms along the wire/tube interface, thus minimizing the total free energy of the system. read less

Abstract: We examine the ramified islands observed in submonolayer Cu/Ni(100) growth. Our results indicate that the strain-energy contribution to the dependence of island energy on shape is surprisingly weak. In contrast, our accelerated dynamics simulations indicate that unexpected concerted popout processes occurring at step edges may be responsible. Kinetic Monte Carlo (KMC) simulations which include these processes produce island shapes which are very similar to those observed in experiment. These results suggest that the shape transition is of kinetic origin but is strongly mediated by strain. read less

Abstract: Plastic pre-strain may decrease the yield strength of metallic materials when stressed in the opposite direction, known as Bauschinger’s effect, which could considerably influence the performance of the materials. However, various processes on microscopic level associated with the Bauschinger’s effect are still not clear. In this study, defect generation, movement and annihilation in single crystal copper during cyclic tension-compression loading processes were simulated using the molecular dynamics method. It was observed that Bauschinger’s effect was asymmetrical and the strain hardening was more profound during compression. After plastic compression, the tensile fracture strain was increased. The absorbed energy was self-adjusted during the tension-compression cycles and kept relatively stable during the loading cycles. Efforts were made to understand the mechanisms responsible for these phenomena. read less

Abstract: Recently a new possibility of welding was experimentally shown in the case of gold nanowires (NWs) at ambient temperatures, without need of additional heat and with low pressures, called cold weldi... read less

Abstract: Fracture of metals at the nanoscale and corresponding failure mechanisms have recently attracted considerable interest. However, quantitative in situ fracture experiments of nanoscale metals are rarely reported. Here it is shown that, under uni‐axial tensile loading, single crystalline ultrathin gold nanowires may fracture in two modes, displaying distinctively different fracture morphologies and ductility. In situ high resolution transmission electron microscopy (HRTEM) studies suggest that the unexpected brittle‐like fracture was closely related to the observed twin structures, which is very different from surface dislocation nucleation/propagation mediated mechanism in ductile fracture mode. Molecular dynamics (MD) simulations further reveal the processes of shear‐induced twin formation and damage initiation at the twin structure/free surface interface, confirming the experimentally observed differences in fracture morphology and ductility. Finally, a fracture criterion based on competition between twin formation and surface dislocation nucleation/propagation as a function of misalignment angle is discussed. read less

Abstract: Vacancies may agglomerate to form vacancy Frank loops of different shapes, as observed by transmission electron microscopy in quenched and irradiated fcc metals. The dynamics for the dissociation of vacancy Frank loops and the subsequent evolution of defect nanostructures were explored by means of the molecular dynamics method and displayed by the local crystalline order method. Frank loops of different initial shapes were found to transform to a variety of defect nanostructures: triangle to stacking fault tetrahedra, equilateral hexagon to quasi-heptahedron, and scalene hexagon to various intermediate structures depending on the length of the short side. The formation energies for vacancy Frank loops of different geometries are introduced to categorize various final configurations. Crystallographic analysis and elasticity calculations were performed to elucidate the transform mechanisms in fcc Ag. read less

Abstract: Many of the most important and hardest-to-solve problems related to the synthesis, performance, and aging of materials involve diffusion through the material or along surfaces and interfaces. These diffusion processes are driven by motions at the atomic scale, but traditional atomistic simulation methods such as molecular dynamics are limited to very short timescales on the order of the atomic vibration period (less than a picosecond), while macroscale diffusion takes place over timescales many orders of magnitude larger. We have completed an LDRD project with the goal of developing and implementing new simulation tools to overcome this timescale problem. In particular, we have focused on two main classes of methods: accelerated molecular dynamics methods that seek to extend the timescale attainable in atomistic simulations, and so-called 'equation-free' methods that combine a fine scale atomistic description of a system with a slower, coarse scale description in order to project the system forward over long times. read less

Abstract: The existence of surface steps was found to have significant function and influence on the growth of graphene on copper via chemical vapor deposition. The two typical growth modes involved were found to be influenced by the step morphologies on copper surface, which led to our proposed step driven competitive growth mechanism. We also discovered a protective role of graphene in preserving steps on copper surface. Our results showed that wide and high steps promoted epitaxial growth and yielded multilayer graphene domains with regular shape, while dense and low steps favored self-limited growth and led to large-area monolayer graphene films. We have demonstrated that controllable growth of graphene domains of specific shape and large-area continuous graphene films are feasible. read less

Abstract: Predicting physical properties of materials with spatially complex structures is one of the most challenging problems in material science. One key to a better understanding of such materials is the geometric characterization of their spatial structure. Minkowski tensors are tensorial shape indices that allow quantitative characterization of the anisotropy of complex materials and are particularly well suited for developing structure‐property relationships for tensor‐valued or orientation‐dependent physical properties. They are fundamental shape indices, in some sense being the simplest generalization of the concepts of volume, surface and integral curvatures to tensor‐valued quantities. Minkowski tensors are based on a solid mathematical foundation provided by integral and stochastic geometry, and are endowed with strong robustness and completeness theorems. The versatile definition of Minkowski tensors applies widely to different types of morphologies, including ordered and disordered structures. Fast linear‐time algorithms are available for their computation. This article provides a practical overview of the different uses of Minkowski tensors to extract quantitative physically‐relevant spatial structure information from experimental and simulated data, both in 2D and 3D. Applications are presented that quantify (a) alignment of co‐polymer films by an electric field imaged by surface force microscopy; (b) local cell anisotropy of spherical bead pack models for granular matter and of closed‐cell liquid foam models; (c) surface orientation in open‐cell solid foams studied by X‐ray tomography; and (d) defect densities and locations in molecular dynamics simulations of crystalline copper. read less

Abstract: Atomistic simulation of Ag, Al, Au, Cu, Ni, Pd, and Pt FCC metallic nanowires show a universal FCC → HCP phase transformation below a critical cross-sectional size, which is reported for the first time in this paper. The newly observed HCP structure is also confirmed from previous experimental results. Above the critical cross-sectional size, initial ⟨100⟩/{100} FCC metallic nanowires are found to be metastable. External thermal heating shows the transformation of metastable ⟨100⟩/{100} FCC nanowires into ⟨110⟩/{111} stable configuration. Size dependent metastability/instability is also correlated with initial residual stresses of the nanowire by use of molecular static simulation using the conjugant gradient method at a temperature of 0 K. It is found that a smaller cross-sectional dimension of an initial FCC nanowire shows instability due to higher initial residual stresses, and the nanowire is transformed into the novel HCP structure. The initial residual stress shows reduction with an increase in the ... read less

Abstract: Diffusion of Cu hexamer islands on Cu(111) and Ag(111) is studied using a molecular dynamics simulation technique with many-body potentials obtained from the embedded atom method. Simulations are carried out at temperatures 300, 500 and 700 K, showing that shape-changing multiple-atom processes are more helpful for the diffusion rather than concerted motion of islands. Arrhenius plots of the diffusion coefficients provide effective energy barrier values of 161.29 ± 5 meV for Cu(111) and 179.34 ± 5 meV for Ag(111) surfaces. At 700K, one popup atom among island atoms is observed with correlative changes in the position and shape of the lower-layer adatoms. read less

Abstract: The effect of temperature on the void nucleation and growth is studied using the molecular dynamics (MD) code LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator). Single crystal copper is triaxially expanded at 5 × 109 s−1 strain rate keeping the temperature constant. It is shown that the nucleation and growth of voids at these atomistic scales follows a macroscopic nucleation and growth (NAG) model. As the temperature increases there is a steady decrease in the nucleation and growth thresholds. As the melting point of copper is approached, a double-dip in the pressure–time profile is observed. Analysis of this double-dip shows that the first minimum corresponds to the disappearance of the long-range order due to the creation of stacking faults and the system no longer has a FCC structure. There is no nucleation of voids at this juncture. The second minimum corresponds to the nucleation and incipient growth of voids. We present the sensitivity of NAG parameters to temperature and the analysis of double-dip in the pressure–time profile for single crystal copper at 1250 K. read less

Abstract: Results of dynamical simulations of collision-induced formation and properties of bimetallic nanoparticles are presented and analyzed. The analysis includes the effects of the collision energy and the impact parameter. For nonzero impact parameters, the formed (in many cases Janus-type) nanoparticles are rotating. The energy of the rotating nanoparticles is decomposed into the rotational and vibrational components, and the structural effects of these components are analyzed. Comparison is made with the case of the corresponding homoatomic systems, formed by collision of nanoparticles with the same elemental composition. read less

Abstract: Modulation of Pd nanoparticle (NP) crystallinity is achieved by switching the surfactants of different binding strengths. Pd NPs synthesized in the presence of weak binding surfactants such as oleylamine possess polyhedral shapes and a polycrystalline nature. When oleylamine is substituted by trioctylphosphine, a much stronger binding surfactant, the particles become spherical and their crystallinity decreases significantly. Moreover, the Pd NPs reconvert their polycrystalline structure when the surfactant is switched back to oleylamine. Through control experiments and molecular dynamics simulation, we propose that this unusual nanocrystallinity transition induced by surfactant exchange was resulted from a counterbalance between the surfactant binding energy and the nanocrystal adhesive energy. The findings represent a novel postsynthetic approach to tailoring the structure and corresponding functional performance of nanomaterials. read less

Abstract: We give an account of recent studies of droplet uptake and withdrawal from carbon nanotubes using simple theoretical arguments and molecular dynamics simulations. Firstly, the thermodynamics of droplet uptake and release is considered and tested via simulation. We show that the Laplace pressure acting on a droplet assists capillary uptake, allowing sufficiently small non-wetting droplets to be absorbed. We then demonstrate how the uptake and release of droplets of non-wetting fluids can be exploited for the use of carbon nanotubes as nanopipettes. Finally, we extend the Lucas-Washburn model to deal with the dynamics of droplet capillary uptake, and again test this by comparison with molecular dynamics simulations. read less

Abstract: On the {1 1 0} solid surface, surface stresses of two perpendicular directions, such as ⟨1 0 0⟩ and ⟨1 1 0⟩, will differ. These different surface stresses cause different equilibrium strains in the ⟨1 0 0⟩ and ⟨1 1 0⟩ directions on a {1 1 0} thin film. In this paper, an analytical formulation is presented to predict the equilibrium strains of orthotropic thin films with {1 0 0}, {1 1 1} or {1 1 0} surfaces. This formulation is composed of the elastic constants of bulk materials and surface parameters such as surface stresses and surface elastic tensors. Transition metals with FCC structures (Cu, Ag, Au, Ni, Pd and Pt) are used in this study, along with a surface relaxation model that calculates surface parameters using atomistic calculations. Molecular dynamics simulations are performed and compared with the results of this suggested method, and both results show good agreement. In addition, size-dependent elastic constants are calculated analytically from the predicted equilibrium state. read less

Abstract: The physical vapour deposition of Ni atoms on α-Fe(001) surface under different deposition temperatures were simulated by molecular dynamics to study the intermixing and microstructure of the interfacial region. The results indicate that Ni atoms hardly penetrate into Fe substrate while Fe atoms easily diffuse into Ni deposition layers. The thickness of the intermixing region is temperature-dependent, with high temperatures yielding larger thicknesses. The deposited layers are mainly composed of amorphous phase due to the abnormal deposition behaviour of Ni and Fe. In the deposited Ni-rich phase, the relatively stable metallic compound B2 structured FeNi is found under high deposition temperature conditions. read less

Abstract: The atomic structure of Pd ultrathin films grown on Ni(111) at 300 K is investigated by low-energy electron diffraction and scanning tunneling microscopy. It is determined atomically that the growth of monolayer Pd films leads to a periodic arrangement of triangular misfit dislocation loops in the underlying Ni(111) surface, resulting in a triangular superstructure on the monolayer Pd surface. The triangular dislocation loops tend to align at an angle of about 5° from the Ni atom row, owing to a slight rotation of the Pd films with respect to the Ni substrate, and appear as a moirelike superstructure on the multilayer Pd surfaces. Atomistic simulations indicate that the slight rotation of monolayer Pd films and the formation of misfit dislocation loops in the Ni surface minimize the Pd–Ni interface energy. read less

Abstract: The accuracy of sales forecast has great impact on manufacturing and sales. With the quick development of the society, it is really hard for traditional forecast system to meet new demand in dealing very large amount of data and sales forecasting with high accuracy. Obviously, data mining technology is the key to solve those problems. In this article, the first two parts are a brief introduction about the concept of Sales Forecast, then an introduction of two popular Extract-Transform-Load (ETL) tools and a comprehensive analysis of four most often used forecast method are given in the following parts. Finally, based on analysis and combined with a variety of technical advantages, we propose a new sales forecast system model based on data mining and Grey-Markov prediction model is used as an example to illustrate its working principle and to verify its feasibility theoretically. read less

Abstract: The present work investigates contributions from surfaces and core nonlinearity to the size-dependent elastic properties of nanowires under bending and tension-compression. When a nanowire is formed by removing it from its parent bulk material, relaxation occurs inevitably because of high energy of newly created surfaces or born high surface eigenstress. Relaxation-induced initial strain could be large and nonlinear, which causes the size-dependent elastic properties of nanowires. If relaxation-induced initial strain is small and linear, the size-dependent elastic properties of nanowires are caused by surface Young’s modulus. The eigenstress model for surface stress of solids {Zhang et al. [Phys. Rev. B 81, 195427 (2010)]} is further developed here for nanowires under bending and tension-compression. The developed eigenstress model leads to general scaling laws for nanowires under bending and tension-compression. In the scaling laws, there are the surface and nonlinearity factors, which measure quantitati... read less

Abstract: In order to establish a link between various structural and kinetic properties of metals and the crystal–melt interfacial mobility, free-solidification molecular-dynamics simulations have been performed for a total of nine embedded atom method interatomic potentials describing pure Al, Cu and Ni. To fully explore the space of materials properties three new potentials have been developed. The new potentials are based on a previous description of Al, but in each case the liquid structure, the melting point and/or the latent heat are varied considerably. The kinetic coefficient, μ, for all systems has been compared with several theoretical predictions. It is found that at temperatures close to the melting point the magnitude of μ correlates well with the value of the diffusion coefficient in the liquid. read less

Abstract: The purpose of this article is to present a multiscale finite element method that captures nanoscale surface stress effects on the dynamic mechanical behavior of nanomaterials. The method is based upon arguments from crystal elasticity, i.e. the Cauchy–Born rule, but significantly extends the capability of the standard Cauchy–Born rule by accounting for critical nanoscale surface stress effects, which are well known to have a significant effect on the mechanics of crystalline nanostructures. We present the governing equations of motion including surface stress effects, and demonstrate that the methodology is general and thus enables simulations of both metallic and semiconducting nanostructures. The numerical examples on elastic wave propagation and dynamic tensile and compressive loading show the ability of the proposed approach to capture surface stress effects on the dynamic behavior of both metallic and semiconducting nanowires, and demonstrate the advantages of the proposed approach in studying the deformation of nanostructures at strain rates and time scales that are inaccessible to classical molecular dynamics simulations. Copyright © 2010 John Wiley & Sons, Ltd. read less

Abstract: In this work we extend classical molecular dynamics by coupling it with an electron transport model known as the two temperature model. This energy balance between free electrons and phonons was first proposed in 1956 by Kaganov et al. but has recently been utilized as a framework for coupling molecular dynamics to a continuum description of electron transport. Using finite element domain decomposition techniques from our previous work as a basis, we develop a coupling scheme that preserves energy and has local control of temperature and energy flux via a Gaussian isokinetic thermostat. Unlike the previous work on this subject, we employ an efficient, implicit time integrator for the fast electron transport which enables larger stable time steps than the explicit schemes commonly used. A number of example simulations are given that validate the method, including Joule heating of a copper nanowire and laser excitation of a suspended carbon nanotube with its ends embedded in a conducting substrate. Published in 2010 by John Wiley & Sons, Ltd. read less

Abstract: This paper consists of two parts. In the first part, the deformation behavior of polycrystalline Cu nanowires under tension, bending and torsion is studied by using molecular dynamics simulations. The results show that both free surfaces and grain boundaries can control the plastic deformation. In detail, for polycrystalline Cu nanowires under tension, the stress-assisted grain growth caused by atomic diffusion and grain boundary migration is found, which is thought to be the reason for necking. Then under the bending effect, full dislocations and deformation twins of polycrystalline Cu nanowires are found in the grains. Moreover, the twin boundaries act as obstacles to dislocation motion. Finally, full dislocations and fivefold deformation twins are detected in polycrystalline Cu nanowires in the torsional state. These phenomena are in good agreement with the experimental observations of Liao et al (2005 Appl. Phys. Lett. 86 103112). The second part investigates the effects of sample shape and crystallographic structure on the modulus and yield strength under tension, bending and torsion. The results demonstrate that the modulus (in particular the bending and torsion modulus) can be significantly influenced by both the effects; however, remarkable difference in yield strength can merely be caused by different crystallographic structures (here, different crystallographic structures refer to polycrystalline and single-crystalline structures). read less

Abstract: We report on the early stages of submonolayer Ag island coarsening on the Ag(111) surface carried out using kinetic Monte Carlo simulations for several temperatures. Our simulations were performed using a very large database of processes identified by their local environment and whose activation barriers were calculated using the semi-empirical interaction potentials based on the embedded-atom method. We find that during the early stages, coarsening proceeds as a sequence of selected island sizes, creating peaks and valleys in the island-size distribution. This island-size selectivity is independent of initial conditions and results from the formation of kinetically stable islands for certain sizes as dictated by the relative energetics of edge atom detachment/attachment processes together with the large activation barrier for kink detachment. Our results indicate that by tuning the growth temperature it is possible to enhance the island-size selectivity read less

Abstract: Solid films are taken here as a typical example to study surface stress of solids. When a thin film is created by removing it from a bulk material, relaxation occurs inevitably because of high energy of newly created surfaces. We separate the relaxation process into normal and parallel relaxations and propose an eigenstress model to calculate the strain energy released during parallel relaxation. After parallel relaxation, a tensile (or compressive) surface eigenstress causes a compressive (or tensile) initial strain in the thin film with respect to its bulk lattice. Due to initial deformation, surface energy density and surface stress are both dependent on the film thickness, whereas surface elastic constants are independent of the film thickness. The nominal modulus of a thin film is determined by nonlinear elastic properties of its core and surfaces with initial strain. A tensile (or compressive) eigenstress makes the nominal modulus of a thin film larger (or smaller), resulting in the thinner, the harder (or softer) elastic behavior in thin films. Atomistic simulations on Au (001), Cu (001), Si (001), and diamond (001) thin films verify the developed eigenstress model. read less

Abstract: Using molecular dynamics, nudged elastic band and modified analytic embedded atom methods, the self-diffusion dynamics properties of palladium atomic clusters up to seven atoms on the Pd (1 1 1) surface have been studied at temperatures ranging from 300 to 1000 K. The simulation time varies from 20 to 75 ns according to the cluster sizes and the temperature ranges. The heptamer and trimer are more stable than the other neighboring clusters. The diffusion coefficients of the clusters are derived from the mean square displacement of the cluster's mass-center, and the diffusion prefactors D0 and activation energies Ea are derived from the Arrhenius relation. The activation energy of the clusters increases with the increasing atom number in the clusters, especially for Pd6 to Pd7. The analysis of trajectories shows the noncompact clusters diffuse by the local diffusion mechanism but the compact clusters diffuse mainly by the whole gliding mechanism, and some static energy barriers of the diffusion modes are calculated. From Pd2 to Pd6, the prefactors are in the range of the standard value 10−3 cm2 s−1, and the prefactor of Pd7 cluster is 2 orders of magnitude greater than that of the single Pd adatom because of a large number of nonequivalent diffusion processes. The heptamer can be the nucleus in the room temperature range according to nucleation theory. read less

Abstract: A coarse graining method that introduces Joule heating and improves heat transport in a classical molecular dynamics simulation is reviewed, and two example sets of simulations, opening of gold–gold nano-asperity contacts and nano-asperity sliding at loaded copper–aluminum interfaces are discussed. For the gold contact, dislocations nucleate from the edges of where the asperity contacts the substrates and move along the close-packed planes, resulting in stacking faults that form two subsurface Thompson tetrahedra. For a null voltage, a nanowire with a diameter much smaller than the initial contact area is created when the two tetrahedra are completed, and as the wire yields the partial dislocations retreat to the surface. Opening with Joule heating enhances dislocation mobility and intransient subsurface plasticity. Constant current simulations show melting and boiling of the nanowires depending on the voltage cap. Sliding of an aluminum asperity on copper with a null voltage shows dislocation formation in the copper and aluminum, while heating from an applied voltage eliminates damage in the copper. Sliding with a copper asperity enhances plastic damage in the copper substrate compared with the aluminum asperity, while Joule heating enhances aluminum pile-up in front of the copper asperity due to plowing. read less

Abstract: We apply molecular dynamics and molecular static methods to study the effect of misfit dislocations on adatom diffusion in close proximity to the dislocation core in heteroepitaxial systems, using many-body interaction potentials. Our system consists of several layers (three–seven) of Cu on top of a Ni(111) substrate. The misfit dislocations are created with the core located at the interface between the Cu film and the Ni substrate, using the repulsive biased potential method described earlier. We find that presence of the defect under the surface strongly affects the adatom trajectory, creating anisotropy in atomic diffusion, independent of the thickness of the Cu film. We also calculate the potential energy surface available to the adatom and compare the energy barriers for adatom diffusion in the proximity of the core region and on the defect-free surface. read less

Abstract: Hydrogen embrittlement is a pervasive mode of degradation in many metallic systems that can occur via several mechanisms. Here, the competition between dislocation emission and cleavage at a crack tip is evaluated in the presence of H. At this level, embrittlement is predicted when the critical stress intensity required for emission rises above that needed for cleavage, eliminating crack tip plasticity and blunting as toughening mechanisms. Continuum predictions for emission and cleavage are made using computed generalized stacking fault energies and surface energies in a model Ni–H system, and embrittlement is predicted at a critical H concentration. An atomistic model is then used to investigate actual crack tip behavior in the presence of controlled arrays of H atoms around the crack tip. The continuum models are accurate at low H concentrations, below the embrittlement point, but at higher H concentrations the models deviate from the atomistic behavior due to alternative dislocation emission modes. Additional H configurations are investigated to understand controlling features of the emission process. In no cases does crack propagation occur in preference to dislocation emission in geometries where emission is possible, indicating that embrittlement can be more complicated than envisioned by the basic brittle–ductile transition. read less

Abstract: Results are reported of a systematic atomic-scale computational analysis of strain relaxation mechanisms and the associated defect dynamics in nanometer-scale thin or ultrathin Cu films that are subjected to a broad range of biaxial tensile strains. The films contain pre-existing voids and the film planes are oriented normal to the [11¯0] crystallographic direction. The analysis is based on isothermal-isostrain molecular-dynamics simulations according to an embedded-atom-method parameterization for Cu and employing multimillion-atom slab supercells. In addition to an initial elastic response for an applied biaxial strain level e<2%, our analysis reveals three regimes in the thin-film mechanical response as e increases. For 2%≤e≤6%, biaxial strain relaxation is dominated by emission and propagation of dislocations (plastic flow) from the surface of the void accompanied by ductile void growth. For 6%read less

Abstract: Atomic scale phenomena driving contact line advancement during the wetting of a solid by a liquid are investigated via molecular dynamics simulations of Ag(l) drops spreading on Ni substrates. For the homologous temperature ∼5% above melting for Ag, essentially non-reactive wetting is observed with relatively high spreading velocity. Analyzing atomic positions with time, including computing flow fields, permits investigation of atomic scale transport mechanisms associated with advancement of the contact line. Delivery of material to the contact line occurs preferentially along the liquid/vapor interface. Ag(l) atoms transported along the liquid/vapor interface become new droplet edge material, effectively displacing existing edge material. Evidence is also shown of a prominent transport and flow mechanism more typically associated with the molecular kinetic theory of spreading: some portion of Ag(l) atoms move along the solid/liquid interface to eventually occupy the contact line region. Selected atomic trajectories are shown to illustrate atoms moving with the contact line, detaching and re-attaching at sites along the solid/liquid interface. However, this latter solid/liquid interface transport mechanism contributed a lower percentage of new material to the advancing contact line compared to the liquid/vapor interface transport mechanism. Features of the AgNi system that may contribute to the dominance of a liquid/vapor interface transport mechanism are highlighted, including a relatively low liquid/vapor surface tension. read less

Abstract: Lattice mismatch of Cu on Ag(111) produces fast diffusion for special "magic sizes" of islands. A size- and shape-dependent reptation mechanism is responsible for low diffusion barriers. Initiating the reptation mechanism requires a suitable island shape, a property not considered in previous studies of 1D island chains and 2D closed-shell islands. Shape determines the dominant diffusion mechanism and leads to multiple clearly identifiable magic-size trends for diffusion depending on the number of atoms whose bonds are shortened during diffusion. read less

Abstract: Towards understanding the effect of solid supports on the catalytic activity of supported metal nanoparticles, all-atom molecular dynamics (MD) simulations are often conducted. However, these calculations are hampered by the uncertainty related to describing metal–support interactions (typically described as Lennard-Jones (LJ) potentials) at the atomic length scale. Ab initio electron-structure calculations are expected to refine such calculations by providing better estimates for the metal–support pair interaction potential. In the case of platinum nanoparticles supported on graphite, recent ab initio results suggest the correct energetic LJ parameter should be about four times that used in previous simulation studies from our group, as well as from others. Stimulated by these findings, MD simulations have been used here to investigate the effect of the magnitude of the metal–carbon interaction on the structure of supported metal nanoparticles. The LJ potential was used to model the metal–carbon interactions, and the embedded-atom method was used to model the metal–metal interactions. The morphology of platinum nanoparticles of 130, 249 and 498 atoms supported on graphite and various bundles of carbon nanotubes (CNTs) was studied. For the larger nanoparticle it was found that, although the details of platinum–carbon interactions are important for correctly capturing the morphological details, the morphology of the support is the primary factor that determines such features. Platinum–carbon interactions affect more significantly the results obtained for metal nanoparticles supported by CNT bundles. In this case, we found that the deviations become significant for small supported nanoparticles, as well as for nanoparticles of any size supported on CNTs of small diameter. read less

Abstract: The introduction of twin boundaries (TBs) within nanocrystalline grains has given scientists an opportunity to enhance mechanical properties that are usually mutually exclusive: strength and ductility. This research is focused on developing a complete understanding of the influences of twin width, grain size and temperature on the deformation characteristics and properties of nanotwinned Cu by large-scale molecular dynamics simulations. Simulation results have shown that a material's toughness can be enhanced by introducing nanotwins, and the enhancement is more pronounced for the higher twin density structures and at lower temperatures. Nanotwinned grains are found to be highly anisotropic in their plastic response; ductile along TBs but strong across them. A random polycrystalline sample gains toughness through the combined response of variously oriented grains. At extremely low temperature, toughness values are elevated further due to depressed dislocation activities inside the grains. The study has also revealed that, unlike twin width refinement, grain size refinement may not always yield superior properties, and may deteriorate material toughness. read less

Abstract: Fil: Ferron, Julio. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Santa Fe. Instituto de Desarrollo Tecnologico para la Industria Quimica. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnologico para la Industria Quimica; Argentina read less

Abstract: We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands form directly during film growth at elevated temperature. The Cr mesas (wedges) form as Cr film steps advance down the staircase of substrate steps, another example of the critical role that substrate steps play in 3D island formation. read less

Abstract: Using atomistic simulations, we show that dislocations efficiently climb in metallic interfaces, such as Cu–Nb, through absorption and emission of vacancies and a counter diffusion of Cu atoms in the interfacial plane. The efficiency of dislocation climb is ascribed to the high vacancy concentration of 0.05 in the interfacial plane, the low formation energy of 0.12 eV with respect to removal or insertion of Cu atoms, and the low kinetic barrier of 0.10 eV for vacancy migration. Dislocation climb facilitates reactions of interfacial dislocations and enables interfaces to be in the equilibrium state with respect to concentrations of point defects. read less

Abstract: We present a review of recent theoretical studies of different atomistic mechanisms of strain relaxation in heteroepitaxial systems. We explore these systems in two and three dimensions using different semi-empirical interatomic potentials of Lennard-Jones and many-body embedded atom model type. In all cases we use a universal molecular static method for generating minimum energy paths for transitions from the coherent epitaxial (defect free) state to the state containing an isolated defect (localized or extended). This is followed by a systematic search for the minimum energy configuration as well as self-organization in the case of a periodic array of islands. In this way we are able to understand many general features of the atomic mechanisms and energetics of strain relaxation in these systems. Finally, for the special case of Pd/Cu(100) and Cu/Pd(100) heteroepitaxy we also use conventional molecular dynamics simulation techniques to compare the compressively and tensilely strained cases. The results for this case are in good agreement with the existing experimental data. read less

Abstract: We review the bond-boost method for accelerated molecular dynamics (MD) simulation and we demonstrate its application to kinetic phenomena relevant to thin-film growth. To illustrate various aspects of the method, three case studies are presented. We first illustrate aspects of the bond-boost method in studies of the diffusion of Cu atoms on Cu(001). In these studies, Cu interactions are described using a semi-empirical embedded-atom method potential. We recently extended the bond-boost method to perform accelerated ab initio MD (AIMD) simulations and we present results from preliminary studies in which we applied the bond-boost method in AIMD to uncover diffusion mechanisms of Al adatoms on Al(110). Finally, a problem inherent to many rare-event simulation methods is the ‘small-barrier problem’, in which the system resides in a group of states connected by small energy barriers and separated from the rest of phase space by large barriers. We developed the state-bridging bond-boost method to address this problem and we discuss its application for studying the diffusion of Co clusters on Cu(001). We discuss the outlook for future applications of the bond-boost method in materials simulation. read less

Abstract: The reorientation mechanism of core–shell nanowires is investigated and our theoretical studies reveal the significance of the structural configuration. In nanowires which have a larger lattice in the core region than in the shell, for example, Au-core and Pd-shell, the surface stress and interfacial stress may synergistically cause them to reorient spontaneously, but they can revert back to the original state upon an appropriate tensile loading. In contrast, the misfit interface is detrimental to spontaneous reorientation in nanowires which have a smaller lattice in the core than in the shell such as the Pd-core and Au-shell structure, but uniaxial tensile loading causes the nanowires to transform in another way. This asymmetrical reorientation is caused by the different intrinsic stress as well as distinctive slipping characteristics, namely partial slipping and perfect slipping in the compressive and tensile processes. read less

Abstract: Three-dimensional molecular dynamics simulations of the nanobending of copper nanowires are carried out. Simulation results show that the loading and unloading cycles of the nanobending test can reveal the full spectrum of the nanowires’ mechanical properties. Up-tensile and bottom-compressive features have been observed along with the neck zone formation. Amorphous region formation is the mechanism of fracture and final breakage. The measured elastic modulus and yield stress are 49 and 7.6 GPa, respectively. Moreover, the effect of the adhesion on the nanobending process is revealed. read less

Abstract: We report developments of the kinetic Monte Carlo (KMC) method with improved accuracy and increased versatility for the description of atomic diffusivity on metal surfaces. The on-lattice constraint built into our recently proposed self-learning KMC (SLKMC) (Trushin et al 2005 Phys. Rev. B 72 115401) is released, leaving atoms free to occupy ‘off-lattice’ positions to accommodate several processes responsible for small-cluster diffusion, periphery atom motion and heteroepitaxial growth. This technique combines the ideas embedded in the SLKMC method with a new pattern-recognition scheme fitted to an off-lattice model in which relative atomic positions are used to characterize and store configurations. Application of a combination of the ‘drag’ and the repulsive bias potential (RBP) methods for saddle point searches allows the treatment of concerted cluster, and multiple- and single-atom, motions on an equal footing. This tandem approach has helped reveal several new atomic mechanisms which contribute to cluster migration. We present applications of this off-lattice SLKMC to the diffusion of 2D islands of Cu (containing 2–30 atoms) on Cu and Ag(111), using the interatomic potential from the embedded-atom method. For the hetero-system Cu/Ag(111), this technique has uncovered mechanisms involving concerted motions such as shear, breathing and commensurate–incommensurate occupancies. Although the technique introduces complexities in storage and retrieval, it does not introduce noticeable extra computational cost. read less

Abstract: In this article, we study the potential of gold nanowires as resonant nanoscale strain sensors. The sensing ability of the nanowires is determined by calculating the variations in resonant frequency that occur due to applied uniaxial tensile and compressive strain. The resonant frequencies are obtained using the surface Cauchy–Born model, which captures surface stress effects on the nanowires through a nonlinear continuum mechanics framework; due to the continuum formulation, the strain-dependent nanowire resonant frequencies are calculated through the solution of a standard finite element eigenvalue problem, where the coupled effects of the applied uniaxial strain and surface stress are naturally included through the finite element stiffness matrix. The nanowires are found to be more sensitive to compressive than tensile strain, with resonant frequency shifts around 200–400 MHz with the application of 1% tensile and compressive strain. In general, the strain sensitivity of the nanowires is found to incre... read less

Abstract: Defects play a key role in the determination of material properties, especially at small scales. The influence of several kinds of defects (point vacancies, line vacancies and cracks) on the deformation and fracture characteristics of a planar copper grain boundary interface with a 45° lattice misorientation is explored using molecular dynamics simulations and the embedded-atom method. Both tensile and shear modes of interfacial separation are considered. The results show that the crystalline defects can have a strong influence on the interfacial behaviours. The sensitivity of the mechanical properties of the interface to a defect type may be different under tension than under shear. It is found that some defect topologies can improve certain properties (e.g. strength and fracture strain) of the bicrystal interface system. read less

Abstract: We report results of a detailed systematic computational analysis of strain relaxation mechanisms and the associated defect dynamics in ultrathin, i.e., a few nanometers thick, Cu films subjected to a broad range of biaxial tensile strains. The analysis is based on isothermal-isostrain molecular-dynamics simulations of the response of Cu films that are oriented normal to the [111] crystallographic direction using an embedded-atom-method parametrization for Cu and multimillion-atom slab supercells. Our analysis reveals five regimes in the thin film’s mechanical response with increasing strain. Within the considered strain range, after an elastic response up to a biaxial strain level e=5.5%, the strain in the metallic thin film is relaxed by plastic deformation. At low levels of the applied biaxial strain above the yield strain (e∼6%), threading dislocation nucleation at the surface of the thin film in conjunction with vacancy cluster formation in the film leads eventually to the formation of voids that ext... read less

Abstract: Molecular dynamics simulations have been used to investigate the morphology and mobility of platinum nanoparticles of various sizes supported by carbon materials. The embedded-atom method was used to model Pt–Pt interactions, and the Lennard-Jones potential was used to model the Pt–C interactions. The C atoms in the supports were held fixed during the simulations. The supports considered were a single graphite sheet and three bundles of carbon nanotubes. Three sizes of Pt nanoparticles were considered: 130 atoms, 249 atoms, and 498 atoms (Pt130, Pt249, and Pt498 respectively). It was found that for all three sizes, diffusion coefficients were approximately one order of magnitude higher for graphite-supported nanoparticles than for carbon nanotube-supported nanoparticles. In addition, increasing the size of the nanoparticle decreased its diffusion coefficient, with Pt130 having the highest and Pt498 the lowest diffusion coefficients. More interestingly, we found that for the Pt nanoparticles of all three sizes the diffusion coefficient increases as temperature increases, reaches a maximum at the melting temperature of the nanoparticle, and then decreases. The melting temperature was found to be strongly dependent on the particle size, but only slightly dependent on the features of the supports. While the size of the nanoparticle was seen to affect the particles’ mobility, it did not significantly affect their structure. The nanoparticles supported by graphite have density profiles that indicate a highly ordered, fcc-like structure, while the particles supported by carbon nanotubes have a more disordered structure. An order parameter confirms that the nanoparticles’ structure depends on the support morphology. read less

Abstract: In nanocrystalline (nc) materials, the misfit of lattice structures at the junctions of grain boundaries (GBs) generally increases local stresses that can be released by GB sliding. In this article, the sliding behaviors of the ordered Σ = 3 (111) GB (twin) and Σ = 11(113) GB in nc palladium have been investigated by means of high-resolution transmission electron microscopy observations and molecular statics calculations. It is found that the twin boundary (TB) can release either large stresses by sliding with a DSC-lattice vector of 1/6 or small stresses by sliding over an imperfect vector. The observed Σ = 11 GB image is exactly symmetrical from which we can clearly identify no GB sliding along the [332] direction, but we cannot determine a sliding component along the direction. The image simulations indicate that the latter is experimentally unidentifiable. read less

Abstract: Department of Low Temperature Physics, Faculty of Mathematics and Physics Charles University in Prague V Holesovickach 2, CZ-180 00 Prague 8, Czech Republic Physique des Solides Irradiés et des Nanostructures CP234 Université Libre de Bruxelles, Bd du Triomphe, B-1050 Brussels, Belgium Faculty of Physics, University of Sofia 5 James Bourchier str., 1164 Sofia, Bulgaria Department of Experimental Nuclear Physics, Physical & Mechanical Faculty K-89, St. Petersburg State Polytechnical University 29 Polytekhnicheskaya str., 195251 St. Petersburg, Russia read less

Abstract: The interactions of hydrogen interstitial and self-interstitial with dissociated Shockley partial dislocations in fcc Ni were studied with embedded atom method calculations and the results were compared with those obtained from elasticity solutions. Such cross-correlations are important for efficient and accurate inclusion of the point defects into the dislocation dynamics simulations that are usually based on elasticity theories. The simulations were carried out using a dipole dislocation cell having periodic boundaries. The size effect, tetragonal distortions and the modulus effect were considered in the elasticity analysis. The results indicate that the elasticity solutions compare well with the atomistic results for the regions outside the Shockley partial cores, even though the interaction energies differed by approximately one order of magnitude for these two types of point defects. The range where the elasticity description of the interstitial–dislocation interaction breaks down was identified. In the self-interstitial case, the core reaction with the interstitial was observed, resulting in a larger core interaction range. read less

Abstract: An Ar+–Ni(1 0 0) collision system at 1 keV impact energy was investigated by using realistic isoenergetic molecular dynamics (MD) simulations. The sputtering process upon Ar+ ion impact and damage to the Ni(1 0 0) surface are examined in detail. Studying of high bombarding energy regions leads to the necessity of larger and thick enough slabs, otherwise incoming ions can easily pass through the slab; as a result, investigated physical properties may not be revealed. In addition the simulation time should be long enough to observe and to calculate a reliable macroscopic property such as sputtering yield that is addressed in this study. In order to preserve the total energy in the simulation at this collision energy a small time-step (0.1 fs) is used. We have made use of our developed linear scaling parallel MD program to overcome these demands. The Ni(1 0 0) slab is formed by 63700 atoms (122 Å × 122 Å × 44 Å) and the total observation time for each collision event is about 2.25 ps. Several properties such as penetration depths, angular and energy distributions of the reflected Ar and sputtered Ni atoms as well as dissociation time, embedded, scattering, sputtering patterns and geometries of the sputtered clusters are also reported, and the calculated sputtering yield is found to be in good agreement with the available experimental results. read less

Abstract: In this paper, we present a study of the phase change processes that take place in Cu and Ni films when they are heated with an electron-beam produced by field emission from an array of carbon nanotubes. A Monte Carlo method is adapted to solve the electron-beam Boltzmann transport equation to determine the electron distribution inside these materials. A hybrid approach is implemented to couple the two-temperature model with molecular dynamics simulations. We consider an analysis based on an order parameter and a radial distribution function to characterize the transition point at which the materials change phase. Slower electron diffusion in Ni produces more pronounced temperature gradients in both the electron system and the lattice, whereas the temperature rise throughout the Cu film is more uniform due to the faster electronic diffusion. We found that the phase change process is a combination of speed of the energy diffusion into the materials accompanied by a concentration of tensile stresses that co... read less

Abstract: A series of molecular dynamics simulations was performed on a bicrystal to which a fixed shear rate was applied parallel to the boundary plane. Under some conditions, grain boundary motion is coupled to the relative tangential motion of the two grains. In order to investigate the generality of this type of coupled shear/boundary motion, simulations were performed for both special (low Σ) and general (non-Σ) [010] tilt boundaries over a wide range of grain boundary inclinations. The data point to the existence of two critical stresses: one for coupled shear/boundary motion and the other for grain boundary sliding. For the non-Σ boundaries, the critical stress for coupled shear/boundary motion is typically smaller than that for sliding; coupled shear/boundary motion occurs for all inclinations. For Σ5 boundaries, for which the critical stress is smaller and depends on boundary inclination, coupled shear/boundary motion occurs for some, but not all inclinations. read less

Abstract: Homogeneous nucleation and growth of zinc from supersaturated vapor are investigated by nonequilibrium molecular dynamics simulations in the temperature range from 400 to 800 K and for a supersaturation ranging from log S=2 to 11. Argon is added to the vapor phase as carrier gas to remove the latent heat from the forming zinc clusters. A new parametrization of the embedded atom method for zinc is employed for the interaction potential model. The simulation data are analyzed with respect to the nucleation rates and the critical cluster sizes by two different methods, namely, the threshold method of Yasuoka and Matsumoto [J. Chem. Phys. 109, 8451 (1998)] and the mean first passage time method for nucleation by Wedekind et al. [J. Chem. Phys. 126, 134103 (2007)]. The nucleation rates obtained by these methods differ approximately by one order of magnitude. Classical nucleation theory fails to describe the simulation data as well as the experimental data. The size of the critical cluster obtained by the mean first passage time method is significantly larger than that obtained from the nucleation theorem. read less

Abstract: Fracture of nanosize contacts formed between spherical probes and flat surfaces is studied using an atomic force microscope in an ultrahigh vacuum environment. Analysis of the observed deformation during the fracture process indicates significant material extensions for both gold and silica contacts. The separation process begins with an elastic deformation followed by plastic flow of material with atomic rearrangements close to the separation. Classical molecular dynamics studies show similarity between gold and silicon, materials that exhibit entirely different fracture behavior at macroscopic scale. This direct experimental evidence suggests that fracture at nanoscale occurs through a ductile process. read less

Abstract: Molecular dynamics simulations have been used to investigate the mobility and morphology of platinum nanoparticles supported on carbonaceous materials. The embedded-atom method was used to model Pt−Pt interactions. The Pt−C interactions were modeled using the Lennard−Jones potential. Carbon atoms were treated as rigid. The supports considered include a single graphite layer as well as carbon nanotubes, regarded as bundles. The goal of our work is to assess the effect of the substrate morphology on the properties of the metal nanoparticles. The properties of interest include the mobility and morphology of the supported nanoparticles. Our results show that the diffusion coefficients of Pt nanoparticles on carbon nanotube bundles are 1 order of magnitude lower than those of Pt nanoparticles supported by graphite. Density profiles, radial distribution functions, and average coordination numbers were calculated to study the structure of the supported nanoparticles. Platinum nanoparticles deposited on carbon na... read less

Abstract: The nature of the regions that are favorable for chemical reactions has been investigated for a pure amorphous water ice substrate following energetic bombardment by C60 and Au3 cluster projectiles using molecular dynamics (MD) simulations. The simulations show that both projectiles, especially C60, produce regions where a plethora of reactions occur at elevated densities indicating that multiple atoms or molecules are involved simultaneously in the reactions initiated by cluster bombardment. The total number of reacted water molecules is significantly less than the total sputtering yield, which confirms that both cluster projectiles are useful for molecular depth profiling experiments. read less

Abstract: The objective of this research is to use atomistic simulations to investigate the energy and structure of symmetric and asymmetric Σ3 ⟨110⟩ tilt grain boundaries. A nonlinear conjugate gradient algorithm was employed along with an embedded atom method potential for Cu and Al to generate the equilibrium 0 K grain boundary structures. A total of 25 ⟨110⟩ grain boundary structures were explored to identify the various equilibrium and metastable structures. Simulation results show that the Σ3 asymmetric tilt grain boundaries in the ⟨110⟩ system are composed of only structural units of the two Σ3 symmetric tilt grain boundaries. The energies for the Σ3 grain boundaries are similar to previous experimental and calculated grain boundary energies. A structural unit and faceting model for Σ3 asymmetric tilt grain boundaries fits all of the calculated asymmetric grain boundary structures. The significance of these results is that the structural unit and facet description of all Σ3 asymmetric tilt grain boundaries may be predicted from the structural units of the Σ3 coherent twin and incoherent twin boundaries for both Cu and Al. read less

Abstract: The useful lifetime of microelectromechanical system switches is shortened during repetitive contact when the continual making and breaking of an electrical circuit accelerates damage done to the metallic contact points in the switch. In this study the interfacial force microscope is used as a model switch, and we explore the fundamental processes involved in switch failure. We find that repeated indentation (cyclic contact) causes protective coatings (in the form of self-assembled monolayers) to fail allowing metal-metal intimacy and formation of a malleable “nanospot weld.” The weld is stretched during separation of the contacting surfaces, leading to the development of nanoasperities. With the help of atomistic simulations, which provide insight into material transfer and consequential roughening of the surfaces, we show that asperity length grows with continued repetition, drastically changing the resistance of the contact over the lifetime of the switch. Controlling the amount of current passed throu... read less

Abstract: The vibrational properties of the Al(001)-c(2 × 2)–Na (Li) ordered phases formed by alkali atoms (Na and Li) on the Al(001) surface at low and room temperatures are presented. The equilibrium structural characteristics, phonon dispersions and polarization of vibrational modes as well as the local density of phonon states are calculated using the embedded-atom method. The obtained structural parameters are in close agreement with experimental data. read less

Abstract: The growth speed of nanoclusters during inert gas condensation has been studied for copper, silver, aluminum, and platinum by using molecular dynamics simulations. We determine the condensation time for vapor atoms in a particular volume to be inversely proportional to the initial partial vapor pressure. We further find that the condensation time depends on the molecular mass and lattice constant, but not on other material properties. An analytical model for the condensation time is derived from kinetic gas theory by using the basic approximations of classical nucleation theory for a homogeneous vapor. read less

Abstract: We examine the metastable liquid phase of a supercooled gold nanocluster by studying the free energy landscape using the largest solidlike embryo as an order parameter. Just below freezing, the free energy exhibits a local minimum at small embryo sizes and a maximum at a larger critical embryo size. At T=660 K the free energy becomes a monotonically decreasing function of the order parameter as the liquid phase becomes unstable, indicating that we have reached a limit of stability. In contrast to the mean-field theory predictions for a spinodal, the size of the critical embryo remains finite as the limit of stability is approached. We also calculate the rate of nucleation, independently from our free energy calculations, and observe a rapid increase in its temperature dependence when the free energy barrier is on the order of kT. We suggest that this supports the idea that freezing becomes a barrierless process at low temperatures. read less

Abstract: The field projection method of Hong and Kim (2003) to identify the crack-tip cohesive zone constitutive relations in an isotropic elastic solid is extended to a nanoscale planar field projection of a cohesive crack tip on an interface between two anisotropic solids. This formulation is applicable to the elastic field of a cohesive crack tip on an interface or in a homogeneous material for any combination of anisotropies. This method is based on a new orthogonal eigenfunction expansion of the elastic field around an interfacial cohesive crack, as well as on the effective use of interaction J integrals. The nanoscale planar field projection is applied to characterizing a crack-tip cohesive zone naturally arising in the fields of atomistics. The atomistic fields analyzed are obtained from molecular statics simulations of decohesion in a gold single crystal along a direction in a (111) plane, for which the interatomic interactions are described by an embedded atom method potential. The field projection provides cohesive traction, interface separation, and the surface-stress gradient caused by the gradual variation of surface formation within the cohesive zone. Therefore, the cohesive traction and surface energy gradient can be measured as functions of the cohesive zone displacements. The introduction of an atomistic hybrid reference configuration for the deformation analysis has made it possible to complete the field projection and to evaluate the energy release rate of decohesion with high precision. The results of the hybrid analyses of the atomistics and continuum show that there is a nanoscale mechanism of decohesion lattice trapping or hardening caused by the characteristics of non-local atomistic deformations near the crack tip. These characteristics are represented by surface relaxation and the development of surface stresses in the cohesive zone. read less

Abstract: Dealloying effects caused by centrifugal forces in rotating multicomponent liquid clusters are investigated with the help of classical molecular dynamics computer simulations. The model system, CuAu, is chosen to investigate the phenomenon as a function of composition and cluster size. In addition to well-established surface energy related effects, we find an enrichment of Au on the surface due to centrifugal forces, which forms a bulge surrounding the rotating cluster. Fluctuations occurring during rotation are found to counteract the dealloying effects by mixing the constituents, leading to a stationary state in the system. read less

Abstract: The different reactions between edge or screw dislocations and interstitial Frank loops were studied by means of molecular dynamics simulations. The calculations were performed at 600 K using an embedded atom method (EAM) potential describing a model FCC material with a low stacking fault energy. An interaction matrix that provides the corresponding interaction strength was determined. In an attempt to investigate the role of pile-ups, simulations with either one or two dislocations in the cell were performed. We find that screw and edge dislocations behave very differently. Edge dislocations shear Frank loops in two out of three cases, while screw dislocations systematically unfault Frank loops by mechanisms that involve cross-slip. After unfaulting, they are strongly pinned by the formation of extended helical turns. The simulations show an original unpinning effect that leads to clear band broadening. This process involves the junction of two screw dislocations around a helical turn (arm-exchange) and the transfer of a dislocation from its initial glide plane to an upper glide plane (elevator effect). read less

Abstract: We present a surface Cauchy-Born approach to modeling FCC metals with nanometer scale dimensions for which surface stresses contribute significantly to the overall mechanical response. The model is based on an extension of the traditional Cauchy-Born theory in which a surface energy term that is obtained from the underlying crystal structure and governing interatomic potential is used to augment the bulk energy. By doing so, solutions to three-dimensional nanomechanical boundary value problems can be found within the framework of traditional nonlinear finite element methods. The major purpose of this work is to utilize the surface Cauchy-Born model to determine surface stress effects on the minimum energy configurations of single crystal gold nanowires using embedded atom potentials on wire sizes ranging in length from 6 to 280 nm with square cross sectional lengths ranging from 6 to 35 nm. The numerical examples clearly demonstrate that other factors beside surface area to volume ratio and total surface energy minimization, such as geometry and the percentage of transverse surface area, are critical in determining the minimum energy configurations of nanowires under the influence of surface stresses. read less

Abstract: We present the results of a comparative study of the equilibrium crystal structure and vibrational properties of the Na/Cu (cid:1) 111 (cid:2) system at coverages up to monolayer saturation. The calculations are performed with interaction potentials from the embedded-atom method. The following ordered structures are considered: p (cid:1) 3 (cid:1) 3 (cid:2) , p (cid:1) 2 (cid:1) 2 (cid:2) , (cid:1) (cid:3) 3 (cid:1) (cid:3) 3 (cid:2) 30°, and (cid:1) 3/2 (cid:1) 3/2 (cid:2) . The surface relaxation, phonon dispersion, and polarization of vibrational modes for the adsorbate and substrate atoms as well as the local density of states are discussed. It is found that the bond length between an adsorbate and the nearest-neighbor substrate atom slightly increases with increasing coverage. Adsorption of sodium also results in a small rumpling in two upper substrate layers. The mode associated with adatom-substrate stretch vibrations was obtained in our calculation at about 22 meV for all the structures considered. The strength of this mode decreases with increasing coverage in accordance with the experiment. On the other hand, we find that the frustrated translation mode frequency of sodium on Cu (cid:1) 111 (cid:2) is strongly coverage dependent. read less

Abstract: A new embedded-atom method (EAM) potential has been constructed for Ag by fitting to experimental and first-principles data. The potential accurately reproduces the lattice parameter, cohesive energy, elastic constants, phonon frequencies, thermal expansion, lattice-defect energies, as well as energies of alternate structures of Ag. Combining this potential with an existing EAM potential for Cu, a binary potential set for the Cu–Ag system has been constructed by fitting the cross-interaction function to first-principles energies of imaginary Cu–Ag compounds. Although properties used in the fit refer to the 0 K temperature (except for thermal expansion factors of pure Cu and Ag) and do not include liquid configurations, the potentials demonstrate good transferability to high-temperature properties. In particular, the entire Cu–Ag phase diagram calculated with the new potentials in conjunction with Monte Carlo simulations is in satisfactory agreement with experiment. This agreement suggests that EAM potentials accurately fit to 0 K properties can be capable of correctly predicting simple phase diagrams. Possible applications of the new potential set are outlined. read less

Abstract: The shape of metallic nanoparticles with diameters in the region of few nanometers is a topic of extreme importantance in nanotechnology and catalysis. The atomic packing of a particle will define its surface orientation and, as a consequence, its catalytic activity, as well as many other properties. In addition, the shape of nanoparticles is strongly dependent on the preparation method and can be in a metastable or even in non-equilibrium state. 9] Dramatic variations in their structure can be understood in terms of the “energy landscape” concept, whereby many structures, or isomers, are represented in a free-energy landscape by minima of similar energies and separated by relatively small energy barriers. This fact is clearly observed in the socalled quasi-melting states, where a nanoparticle changes its shape in a very dramatic way under an electron beam, an effect that has been extensively studied by several authors and is a direct proof of the energy landscape concept. 9] When metallic nanoparticles are synthesized, their three-dimensional (3D) shape or structure results in 2D projections in the transmission electron microscopy (TEM) image corresponding to different shapes such as squares, triangles, hexagons, quasi-circles, and pentagons. These shapes can be associated in three dimensions to the cube, tetrahedron, cubo-octahedron, icosahedron, and decahedron, respectively, depending on the orientation of the particles on the substrate surface. When the particles are of 1–2 nm size, it is possible to observe the fivefold orientation more frequently, and this is probably due to truncations on the surface in contact with the substrate. Also, when bimetallic nanoparticles are synthesized, the frequency of the icosahedral particle increases, particularly for the smallest sizes. Platonic solids are polyhedra formed by the faces made of a same regular polygon. In total there exist five Platonic solids, namely, the tetrahedron, cube, octahedron, dodecahedron, and icosahedron. Composed of regular triangles, there exists the tetrahedron, octahedron, and icosahedron with 4, 8, and 20 faces, respectively. The cube, with 6 faces, is the only Platonic solid composed of squares, and the dodecahedron, with 12 faces, is the only one composed of pentagons. No other solid can be constructed from regular polygons. This is the reason that ancient Greeks attributed five shapes of special significance to the five elements (fire, earth, water, air, and cosmos), believing they must constitute the basic building blocks of the universe. Interestingly, today:s modern technology has confirmed that many nanoparticles take on some Platonic-solid-related shapes. Each Platonic solid looks the same from any vertex, and intuitively they appear as good candidates for shapes found at atomic equilibrium. A very clear example is the icosahedral (Ih) particle, which only exhibits (111) facets that contribute to produce a more rounded structure. Indeed, many studies report the Ih form as the most stable particle at the size range r 20 ; for noble gases and for some metals. However, in Platonic-solid clusters of a particular shape, an internal strain builds up with size; at some point the surface tension is suppressed in expense of strain energy and a new shape is favored. In that limit two different faces appear and an Archimedean solid is generated. A very elegant analytical treatment of the transition between an icosahedron (Platonic) to a Wulff polyhedron (Archimedean) has been presented by Nagaev, and we refer the reader to that article for more details. This transition is size-dependent and for noble metals, r 20 ; corresponds to an optimum value for the transition. The other Platonic solid structures such as tetrahedra, cubes, and octahedra have also been reported, but surprisingly dodecahedral structures have not been reported in this small size range. In this Communication we report the structure of small bimetallic nanoparticles of size r 2 nm. The profiles that these binary particles show in the microscope do not match with any of the known profiles in this size range. Therefore, we propose some very likely structures, based on the dodecahedron, with a close resemblance to the experimental profiles. We present some detailed models based on dodecahedral atomic growth packing, their corresponding HRTEM simulations, as well as their energetic stability through ab initio and embedded-atom method (EAM) calculations in order to discern/assign the structure for these binary AuPd nanoparticles. [*] Dr. J. M. Montejano-Carrizales Instituto de F sica, Universidad Aut noma de San Luis Potos 78000 San Luis Potos (Mexico) read less

Abstract: In the present work, a new way to obtain bimetallic nanoclusters of different structures and chemical compositions is proposed, which is based on computer simulations. Collision processes between two metal clusters of different natures are simulated through molecular-dynamics simulations using many-body potentials. Diverse diffusion mechanisms and structures can be observed, depending on the metals combined and the initial kinetic energies. The nanostructures we have found are core-shell (Pt-Au), alloyed (Pd-Au), and three-shell onionlike (Cu-Ag). read less

Abstract: Atomic-scale simulations are used to examine the plastic behaviour of copper multi-layered thin films during nanoindentation tests. It is found that glide of nucleated dislocation loops and slip in the grain boundaries are the main operating deformation mechanisms in such multi-layered polycrystals. Furthermore, for a very small layer thickness, slip in the grain boundary dominates over the dislocation mediated plasticity. In order to survey the resistance of a multi-layered metal thin film to plastic deformation during nanoindentation, hardness versus penetration depth curves are plotted. Hardness curves reveal softening of multi-layer films, i.e. a reverse Hall–Petch effect is found for layer thicknesses in the nanometre range. read less

Abstract: We present a methodology for the accurate and efficient extraction of elastic constants in homogeneous solids via the calculation of the atomic displacement correlation function. This approach is validated for cubic solids parametrized by both Lennard-Jones and embedded-atom method potentials. Finally, we also discuss the extension of this method to obtain the elastic properties of inhomogeneous solids. read less

Abstract: Molecular dynamics simulations are used to examine static and dynamic coexistence between solid and liquid phases in nanoscale silver, copper, and nickel clusters. We find static coexistence in the 561-atom copper icosahedron, the 561-atom silver icosahedron, and the 923-atom nickel icosahedron, and in cluster sizes above these thresholds, but not in smaller clusters. Nonetheless, in smaller clusters we typically observe either dynamic coexistence between fully solid and liquid states or transient coexistence which is essentially dynamic coexistence between a fully solid state and a solid-liquid state. read less

Abstract: A comprehensive computational analysis is reported of the atomistic mechanisms of strain relaxation and failure in free-standing Cu thin films under applied biaxial tensile strain for strain levels up to 6%. The analysis focuses on nanometer-scale-thick films with a preexisting void extending across the film thickness and the film plane oriented normal to the [111] crystallographic direction. Our computational study is based on isothermal-isostrain large-scale molecular-dynamics simulations within an embedded-atom-method parametrization for Cu. Our analysis has revealed various regimes in the film’s mechanical response as the applied strain level increases. Within the considered strain range, after an elastic response at a low strain (<2%), void growth is the major strain relaxation mechanism mediated by the emission of perfect screw dislocation pairs from the void surface and subsequent dislocation propagation; as a result, a plastic zone forms around the void. Plastic deformation is accompanied by the g... read less

Abstract: The role of the crystalline orientation of the STM tip in the generation of metal clusters is studied by atom dynamics simulations. When a (111) facet is facing the surface, the process is accompanied by a perturbation of the surface stronger than that observed for more open tip structures. This implies a technological application: the possibility of orienting a nanocrystallite deposited on a tip according to the changes observed in the force on the tip. read less

Abstract: We investigated the vibrational and structural properties of the Al s 111 d - s ˛ read less

Abstract: An efficient approach is presented for performing efficient molecular dynamics simulations of solute aggregation in crystalline solids. The method dynamically divides the total simulation space into "active" regions centered about each minority species, in which regular molecular dynamics is performed. The number, size, and shape of these regions is updated periodically based on the distribution of solute atoms within the overall simulation cell. The remainder of the system is essentially static except for periodic rescaling of the entire simulation cell in order to balance the pressure between the isolated molecular dynamics regions. The method is shown to be accurate and robust for the Environment-Dependant Interatomic Potential (EDIP) for silicon and an Embedded Atom Method potential (EAM) for copper. Several tests are performed beginning with the diffusion of a single vacancy all the way to large-scale simulations of vacancy clustering. In both material systems, the predicted evolutions agree closely with the results of standard molecular dynamics simulations. Computationally, the method is demonstrated to scale almost linearly with the concentration of solute atoms, but is essentially independent of the total system size. This scaling behavior allows for the full dynamical simulation of aggregation under conditions that are more experimentally realizable than would be possible with standard molecular dynamics. read less

Abstract: Dislocation half-loops intersecting the surface have been detected by Scanning Tunneling Microscopy after mild sputtering on Aus100d. We determine their structure and formation energy by means of atomistic simulations. The Peierls barrier is also estimated from the simulations and found to be a few meV. This represents a very efficient way of moving mass parallel to the surface under applied stress, where a net movement of hundreds of atoms can take place with barriers smaller than that for the diffusion of a single adatom. Thermal diffusion is not observed. read less

Abstract: Ab initio simulations are performed for Cu atoms adsorbed on the perfect MgO(001) substrate, with an ordered metal coverage varied from 1 monolayer (ML), i.e. almost single atoms, up t o1M L. As trong dependence of the adhesion energy and the sub-monolayer film distance from the substrate on the surface coverage and adsorbate positions (Mg 2+ or O 2− )i s discussed. The nature of interfacial bonding at all coverages is physisorption .W hen increasing Cu atomic fraction, a decrease of the substrate-induced polarization of adatoms accompanied by an increase of both in-plane metallic bonding and the interfacial distance has been found. Combining results of ab initio calculations with thermodynamic theory (taking into account the lattice mismatch), we show that the metal cluster formation becomes the predominant growth mode even at low Cu coverages, in agreement with experiment. (Some figures in this article are in colour only in the electronic version) read less

Abstract: Metal nanoclusters can be produced cheaply and precisely in an electrochemical environment. Experimentally this method works in some systems, but not in others, and the unusual stability of the clusters has remained a mystery. We have simulated the deposition of the clusters using classical molecular dynamics and studied their stability by grand-canonical Monte Carlo simulations. We find that electrochemically stable clusters occur only in those cases where the two metals involved form stable alloys. read less

Abstract: Recent theoretical results indicate that ion-beam mixing can be used to synthesize nanocomposite structures from immiscible elements, relying on a self-organization phenomenon at steady state under irradiation. According to this modeling, self organization requires that the range of the forced atomic relocations in displacement cascades exceeds a critical value. Experimental evidence supporting the formation of nanocomposites by this mechanism has been found in the immiscible system Ag‐Cu irradiated with 1 MeV Kr ions. To address this experimentally relevant model system, and to test the theoretical predictions, we study, by molecular dynamics ~MD!, the characteristics of irradiation mixing in a Ag‐Cu alloy subjected to bombardment with 62 keV He, 270 keV Ne, 500 keV Ar, and 1 MeV Kr ions. The distribution of atomic relocations measured by MD is then used to perform lattice kinetic Monte Carlo ~KMC! simulations of phase evolution, during which both thermal decomposition and irradiation mixing operate simultaneously. The KMC results show that, in the framework of this self-organization mechanism, a nanocomposite structure can be stabilized at steady state by irradiation with heavy ions ~Ne, Ar, and Kr!, but not with He ions. As the characteristic relocation range for He ions is half of that measured for the heavy ions, these results support the theoretical prediction of the existence of a critical relocation range for compositional patterning to take place under irradiation. © 2003 American Institute of Physics. @DOI: 10.1063/1.1540743# read less

Abstract: The properties of materials (mechanical, electronic, magnetic, etc) derive ultimately from the identity and spatial arrangement of their constituents. Nowadays, with the dimensions of technological devices and nanostructures reaching a few atomic constants, descriptions in terms of macroscopic concepts appear to be frequently inadequate and must give way to atomistic formulations based on elementary processes. Focusing on metallic materials, and more specifically on low-dimensional systems such as ultrathin films, superlattices or nanostructures, this paper reviews the atomic scale phenomena responsible for the most common types of defects (interfacial alloying, etching and roughness, formation of dislocations and pinholes, film discontinuities and twinning). It is shown that many of these features are related to the different mechanisms of strain relaxation in heteroepitaxial systems as well as to specific characteristics of atomic diffusion, such as the presence of Ehrlich–Schwoebel barriers hindering step crossings. Some special growth techniques (use of surfactants and codeposition) are also presented together with experimental examples demonstrating their usefulness to overcome the elements' natural limitations and produce accurately controlled, custom-designed epitaxial samples. Finally, a brief overview is given of different phenomena that can be exploited to produce self-assembled or self-organized structures. read less

Abstract: The properties of palladium clusters, generated with the electrochemical scanning tunneling microscope, have been investigated both by experiments and by computer simulations. The clusters are found to be larger and more stable if the tip is moved further towards the electrode surface in the generation process. The simulations suggest that the larger clusters consist of a palladium–gold mixture, which is more stable than pure palladium. Dissolution of the clusters occurs from the edges rather than layer by layer. read less

Abstract: We have studied the atomic arrangement and defect formation in metal nanowires (NWs) generated by mechanical elongation using in situ high resolution transmission electron microscopy. It has been observed that the narrowest constriction of gold and platinum NWs is crystalline and defect-free; in particular, gold NWs adopt only three kinds of atomic arrangement. A model correlating these gold structures and the quantum conductance behaviour is proposed, which showed a remarkable agreement with ultrahigh-vacuum mechanically controllable break junction electrical transport measurements. read less

Abstract: We present the Step and Slide method for finding saddle points between two potential-energy minima. The method is applicable when both initial and final states are known. The potential-energy surface is probed by two replicas of the system that converge to the saddle point by following isoenergetic surfaces. The value of the transition-state potential is bracketed in the process, such that a convergence criterion based on the potential can be used. We applied the method to study diffusion mechanisms of a small Ag cluster on a Ag(111) surface using an embedded-atom method potential. Our approach is comparable in efficiency to other commonly used methods. read less

Abstract: The temperature dependence of EXAFS (extended X-ray absorption fine structure) cumulants was investigated for bulk Cu and a thin film of Cu by means of the path-integral effective classical potential method. By using the semi-empirical embedded-atom method as a potential, agreement between the experiments and calculations is found to be excellent for bulk Cu. In the thin Cu(111) film, anisotropic anharmonic vibration was clearly observed; the out-of-plane vibration is much more enhanced and more anharmonic than the lateral vibration. The results are semiquantitatively consistent with the previous experimental data on Cu(111)/graphite. Such a vibrational enhancement should be the driving force for the roughening transition and/or the surface pre-melting at higher temperature. The practical usefulness of the path-integral effective classical potential method combined with the embedded-atom method is demonstrated. read less

Abstract: The migration energies EM for the jumps into nearest-neighbour vacancies in L12 intermetallic compounds are related to the static lattice Green functions that can be calculated from the measured phonon dispersions. The present approach is an extension of a similar approach used earlier for BCC and FCC pure metals (Schober H R, Petry W and Trampenau J 1992 J. Phys.: Condens. Matter 4 9321). In A3B compounds with the L12 structure, three first-nearest-neighbour jumps into vacancies have to be distinguished and in some cases there is a bias, ΔE, between final and initial configurations. As was done earlier for monatomic lattices by Schober et al, the migration energy is split into two terms, one depending only on structure and a material-dependent term, given by the Green function elements. The difference in size between A and B atoms has to be taken into account and the compounds have to be separated into two groups depending on the size of the majority atoms relative to the minority ones. The formulae have been checked by computer simulations. The values of EM or EM-ΔE/2 are calculated for those L12 compounds where the phonon dispersions have been measured. In the case of the Ni3Al system, other theoretical and experimental determinations compare well with our model. Comparing these energies with the critical temperatures of stability of the L12 structure, we note a significant contribution of the ordering energy to the migration energy for all three jump types. read less

Abstract: A molecular-dynamics study is presented of the mechanism and kinetics of void growth and morphological evolution in ductile metallic thin films subject to biaxial tensile strains. The void becomes faceted, grows, and relieves strain by emission from its surface of pairs of screw dislocations with opposite Burgers vectors. Repeated dislocation generation and propagation leads to formation of a step pattern on the film’s surfaces. A simple phenomenological kinetic model of void growth is derived. Such kinetic equations can be used to formulate constitutive theories of plastic deformation for continuum-scale modeling of void evolution. read less

Abstract: The properties of multilayered materials are often dependent upon their interfacial structure. For low temperature deposition processes where the structure is kinetically controlled, the interfacial roughness and the extent of interlayer mixing are primarily controlled by the adatom energy used in the deposition. Inert gas ion assistance during the growth process also enables manipulation of the interfacial roughness and intermixing. To explore inert gas ion assistance, a molecular dynamics approach has been used to investigate the role of ion energy and ion species upon the flattening of various surfaces formed during the growth of the Ni/Cu/Ni multilayers. The results indicated that ion energies in the 1–4 eV range could flatten the “rough” copper islands on either copper or nickel crystals. To flatten the rough nickel islands on copper or nickel crystals, higher ion energies in the 9–15 eV range would have to be used. Significant mixing between nickel island atoms and the underlying copper crystal atom... read less

Abstract: Recent molecular dynamics simulations of high-energy high-angle twist grain boundaries (GBs) in Si revealed a universal liquid-like high-temperature structure which, at lower temperatures, undergoes a reversible structural and dynamical transition from a confined liquid to a solid; low-energy boundaries, by contrast, were found to remain solid all the way up to the melting point. Here we demonstrate for the case of palladium that fcc metal GBs behave in much the same manner. Remarkably, at high temperatures the few representative high-energy high-angle (tilt or twist) boundaries examined here exhibit the same, rather low self-diffusion activation energy and an isotropic liquid-like diffusion mechanism that is independent of the boundary misorientation. These observations are in qualitative agreement with recent GB self- and impurity-diffusion experiments by Budke et al. on Cu. Our simulations demonstrate that the decrease in the activation energy at elevated temperatures is caused by a structural... read less

Abstract: The energetic atom deposition of thin Au/Au(100) film has been studied by molecular dynamics simulation using the Au-Au interatomic interaction potential with embedded atom method. By investigating the variation of coverage curves and Bragg diffraction intensities during the film growth, the transition of Stranski-Kranstanov growth mode to Frank-van der Merwe growth mode was observed with the increase of the incident energy of deposition atoms. The role of energetic atoms in the film growth is discussed by analyzing the transport properties of deposited atoms and the evolution of incident energy and substrate temperatures. read less

Abstract: The multilayer relaxation at (100), (110), (111), (210), (211), (310), (311) and (331) surfaces of the fcc metals Cu, Ag, Au, Ni, Pd, Pt, Al and Pb are calculated using the modified embedded atom method. The `anomalous' outward relaxation at Al (100), (111), Pt (111), and Cu (111) surfaces is described correctly. The relief of surface stress and tension on the relaxation is studied on (111), (100) and (110) surfaces. In general, the surface stress in the direction of the surface normal determines the relaxation direction except for the Al (110) surface. When the surface stress is negative, the surface relaxation is inward; otherwise, the relaxation is outward. An interesting result is that the surface tension does not always decrease after relaxation. The outward relaxation will induce the increase in surface tension while the inward relaxation induces the decrease in surface tension. read less

Abstract: This study investigates the effect of the Pt/Pd ratio on the oxidation activity and sulfur poisoning/regeneration of diesel oxidation catalysts (DOC) using beta zeolites with high siliceous content as support. Formation of Pt-Pd alloy leads to contraction of the cell lattice of Pt in the bimetallic catalysts, improving not only the sintering resistance of Pt but also retaining a high fraction of Pd in Pd 2 + form. Moreover, the Pt-Pd alloy also improves the oxidation resistance of the particles, which enhances the activity of the catalysts for CO and C 3 H 6 oxidation. Bimetallic catalysts also favor NO reduction at a lower temperature than the monometallic Pt although they showed lower values for the absolute conversion of NO due to a decrease in the total number of the Pt active sites. In addition, the bimetallic catalysts significantly improved the sulfur resistance as compared to the monometallic Pd catalyst. Moreover, the bimetallic catalysts could easily recover their activity for NO and C 3 H 6 oxidation by thermal treatment either in lean conditions or in H 2 . The reduction with H 2 was necessary to recover completely the activity of the CO and C 3 H 8 oxidation. read less

Abstract: A detailed study of the mobility and kinetic properties of solid liquid interfaces (SLI) with different types of crystal lattices (fcc Al, Cu) and (bcc Fe) metals in a wide range of temperatures and pressures was carried out using atomistic modeling. The ranges of maximum permissible values of superheated/undercooled states for each metal have been determined. The ultimate goal of the study was to determine the temperature dependences of the stationary front velocity υsl(ΔT) describing the SLI mobility in each of the metals in an analytical form. The analytical dependence υsl(ΔT) was constructed by comparing the results of atomistic modeling in the area of maximum permissible superheating/undercooling values with the data of the main kinetic models of Wilson Frenkel (WF) and Broughton, Gilmer and Jackson (BGJ). An acceptable agreement was achieved by introducing appropriate correction parameters into the kinetic models using the least squares method. The influence of the crystallographic orientation of metals and external pressure on the SLI mobility is investigated. read less

Abstract: Nanoclusters have gained a huge interest due to their unique properties. They represent an intermediate state between an atom and a solid, which manifests itself in their atomic configurations and ... read less

Abstract: Anisotropic energy transport properties were determined theoretically for crystals of the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) using molecular dynamics simulations. Determination of these properties is necessary to predict and understand processes such as shock response and hot spot formation/relaxation and is also important for accurate parameterization of engineering models. Many properties of TATB exhibit significant anisotropy, which is thought to be due to the triclinic, graphitic-like layered packing structure. The thermal conductivity was determined as a function of temperature, pressure, and defect density; conduction within the layers is approximately 70% greater than conduction between the layers. Hot spot relaxation simulations were compared with and fit to solutions for the 1D heat equation to assess the validity of using continuum models to describe heat transport in TATB on nanometer length scales. A dissipative particle dynamics at constant energy coarse-grained model was developed for TATB and applied to micron-scale shock simulations. The predicted shock response is shown to be highly sensitive to a model parameter controlling energy transport kinetics, underscoring the need for a physics-based upscaling approach. A generalized crystal-cutting method was developed to construct 3D periodic simulation cells containing arbitrarily oriented single crystals and crystalcrystal interfaces for materials of arbitrary symmetry class. Strategies for non-uniform sampling of simulated transient phenomena were proposed that drastically reduce data storage costs. read less

Abstract: Within the harmonic approximation, the analytic expression of the dynamical matrix is derived based on the modified analytic embedded atom method (MAEAM) and the dynamics theory of surface lattice. The surface phonon dispersions along three major symmetry directions , and are calculated for the clean Ag (100) surface by using our derived formulas. We then discuss the polarization and localization of surface modes at points and by plotting the squared polarization vectors as a function of the layer index. The phonon frequencies of the surface modes calculated by MAEAM are compared with the available experimental and other theoretical data. It is found that the present results are generally in agreement with the referenced experimental or theoretical results, with a maximum deviation of 10.4%. The agreement shows that the modified analytic embedded atom method is a reasonable many-body potential model to quickly describe the surface lattice vibration. It also lays a significant foundation for studying the surface lattice vibration in other metals. read less

Abstract: OF THE DISSERTATION Continuum and atomistic models of surface elasticity and applications by Lixin Hu Dissertation Director: Professor Liping Liu We present an analysis of surface elasticity from the Born-Oppenheimer approximation for monatomic crystals. The analysis shows that the relaxations of crystal planes parallel to a free surface can be sufficiently determined by a low-rank algebraic Riccati equation instead of a full-scale molecular dynamic (MD) simulation, and gives new restrictions on physically reasonable atomistic models and simple criteria for surface reconstructions. In the case of surface relaxations, we calculate surface elasticity properties from atomistic models, which are compared with experimental data and prior simulation results. This fundamental research is useful in a variety of applications. First, with the help of the proposed algorithm we quickly calculate the surface tension and determine the equilibrium shape of crystals. Secondly, in previous studies of wave propagation the impact of surface elasticity was not noticed. We find that when the surface/interface gains its own elasticity, the inhomogeneities between the bulk and the surface/interface result in nonlinearity for both interfacial and bulk wave propagation aspects. We study the interfacial wave between two half-spaces with surface elasticity taken into account. read less

Abstract: Recent studies of the dynamics of diverse condensed amorphous materials have indicated significant heterogeneity in the local mobility and a progressive increase in collective particle motion upon cooling that takes the form of string-like particle rearrangements. In a previous paper (Part I), we examined the possibility that fluctuations in potential energy E and particle mobility μ associated with this 'dynamic heterogeneity' might offer information about the scale of collective motion in glassy materials based on molecular dynamics simulations of the glassy interfacial region of Ni nanoparticles (NPs) at elevated temperatures. We found that the noise exponent associated with fluctuations in the Debye-Waller factor, a mobility related quantity, was directly proportional to the scale of collective motion L under a broad range of conditions, but the noise exponent associated with E(t) fluctuations was seemingly unrelated to L. In the present work, we focus on this unanticipated difference between potential energy and mobility fluctuations by examining these quantities at an atomic scale. We find that the string atoms exhibit a jump-like motion between two well-separated bands of energy states and the rate at which these jumps occur seems to be consistent with the phenomenology of the 'slow-beta' relaxation process of glass-forming liquids. Concurrently with these local E(t) jumps, we also find 'quake-like' particle displacements having a power-law distribution in magnitude so that particle displacement fluctuations within the strings are strikingly different from local E(t) fluctuations. An analysis of these E(t) fluctuations suggests that we are dealing with 'discrete breather' excitations in which large energy fluctuations develop in arrays of non-linear oscillators by virtue of large anharmonicity in the interparticle interactions and discreteness effects associated with particle packing. We quantify string collective motions on a fast caging times scale (picoseconds) and explore the significance of these collective motions for understanding the Boson peak of glass-forming materials. read less

Abstract: In this paper we present our results of simulating a pull-out test of single walled carbon nanotubes (SWCNT) out of a single crystal gold lattice by means of molecular dynamics. We compare the obtained force-displacement data of the pullout test to results of simulated uniaxial tensile strain tests of SWCNTs. In doing so, we make a theoretical estimation about the quality of the clamping of SWCNTs in a gold crystal. We investigated the influence of chirality of SWCNTs and of the system temperature. Dependent on SWCNT chirality two different pull-out behaviours can be described. Zigzag nanotubes show stronger pull-out resistance than chiral or armchair nanotubes. Our results indicate a minor influence of embedding length of the SWCNT in the gold matrix on pull-out forces. The system temperature has only little effect on the maximum pull-out forces. The presented results have impact on design criteria of SWCNT-metal interfaces. read less

Abstract: High velocity impact of copper plates using molecular dynamics has been performed to study the spallation of single crystal copper at impact velocities of 1100 and 1000 m s−1. The molecular dynamics code LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator) with the embedded atom method potential is used for this study. It is found that for an impact velocity of 1100 m s−1, nucleation and growth of multiple voids take place which lead to the spallation of the material. For the impact at 1000 m s−1 in the ⟨1 0 0⟩ impact direction, the material does not undergo spallation but gives a spall-like signal in the free surface velocity of the target. We show that the tension developed by first traversal of the shock wave creates various kinds of defects in the target. These become void nucleation sites during the subsequent traversal of the shock wave. The presence of void nucleation sites due to the first traversal of the shock leads to the nucleation of the voids at a lower tensile pressure. We also show that the spall-like signal in the free surface velocity of the target at 1000 m s−1 impact along the ⟨1 0 0⟩ direction occurs due to stress relaxation resulting from the nucleation and growth of the voids without physical separation of scab from the target. read less

Abstract: Molecular dynamics computer simulations are used to probe the development of the surface morphology and the processes that determine the depth resolution in depth profiling experiments performed by secondary ion and neutral mass spectrometry (SIMS/SNMS). The Ag(111) surface is irradiated by an impact of 20‐keV Au3, C60 and Ar872 clusters that represent a broad range of cluster projectiles used in SIMS/SNMS experiments. Improvements in the simulation protocol including automation and optimal sample shape allow for at least 1000 consecutive impacts for each set of initial conditions. This novel approach allows to shrink the gap between single‐impact simulations and real experiments in which numerous impacts are used. Copyright © 2010 John Wiley & Sons, Ltd. read less

Abstract: The room temperature deposition of copper onto a Au(111)-(22×√3) reconstructed surface has been investigated using scanning tunnelling microscopy (STM), up to a copper coverage of approximately 0.7 monolayer (ML). At extremely low coverage (∼0.02 ML), preferential adsorption is observed to occur by displacement of gold atoms and incorporation of copper into the top gold layer at alternate herringbone elbows along the ⟨112⟩ directions. Both fcc regions and hcp regions are occupied. With increasing coverage, incorporation of copper continues but copper is also deposited on top of the incorporated copper islands. When full coverage of these islands to monolayer thickness is reached, further deposition leads to preferential growth of those islands located in hcp regions through both the deposition process and migration of copper from other elbows, predominantly those in fcc regions. Eventually, a critical island size is reached above which atomically thick copper islands exhibit a reconstructed surface similar, in essence, to that of the clean gold surface. Models for the initial adsorption mechanism, island formation and the eventual reconstruction of the copper islands are discussed qualitatively in terms of surface strain within the gold and copper surfaces. read less

Abstract: This study is made to elucidate the primary mechanism dominating the inception and progress of eutectic melting between two solid metals contacting with each other. In this study, Cu-Ag eutectic system is simulated with classical molecular dynamics using Embedded Atom Method (1) . First, melting temperature of solid solution of Cu-Ag binary system is investigated. The melting temperature depends on the atomic concentration of Cu and follows the liquidus line obtained in the experiments. The minimum melting temperature was obtained at the eutectic concentration. The melting behavior on the interface between two pure Cu and Ag slabs are then simulated. The mutual diffusion at the interface was considerably enhanced by the surface melting of both the metals. It is shown that the melting temperature at the interface is lowered depending on the local value of the atomic fraction and is almost identical to that of the solid solution with the corresponding atomic fraction. read less

Abstract: The generation and dissipation of latent heat at the moving solid–liquid boundary during non-equilibrium molecular dynamics (MD) simulations of crystallization can lead to significant underestimations of the interface mobility. In this work we examine the heat flow problem in detail for an embedded atom description of pure Ni and offer strategies to obtain an accurate value of the kinetic coefficient, μ. For free-solidification simulations in which the entire system is thermostated using a Nose–Hoover or velocity rescaling algorithm a non-uniform temperature profile is observed and a peak in the temperature is found at the interface position. It is shown that if the actual interface temperature, rather than the thermostat set point temperature, is used to compute the kinetic coefficient then μ is approximately a factor of 2 larger than previous estimates. In addition, we introduce a layered thermostat method in which several sub-regions, aligned normal to the crystallization direction, are indepently thermostated to a desired undercooling. We show that as the number of thermostats increases (i.e., as the width of each independently thermostated layer decreases) the kinetic coefficient converges to a value consistent with that obtained using a single thermostat and the calculated interface temperature. Also, the kinetic coefficient was determined from an analysis of the equilibrium fluctuations of the solid–liquid interface position. We demonstrate that the kinetic coefficient obtained from the relaxation times of the fluctuation spectrum is equivalent to the two values obtained from free-solidification simulations provided a simple correction is made for the contribution of heat flow controlled interface motion. Finally, a one-dimensional phase field model that captures the effect of thermostats has been developed. The mesoscale model reproduces qualitatively the results from MD simulations and thus allows for an a priori estimate of the accuracy of a kinetic coefficient determination for any given classical MD system. The model also elucidates that the magnitude of the temperature gradients obtained in simulations with a single thermostat depends on the length of the simulation system normal to the interface; the need for the corrections discussed in this paper can thus be gauged from a study of the dependence of the calculated kinetic coefficient on system size. read less

Abstract: Pt is the most common catalyst for NO oxidation to NO2, a key reaction in NOx remediation chemistry. In this work, density functional theory calculations and plane-wave supercell models are used to calculate the energies, charge distributions, and vibrational spectra of the stable and metastable states of adsorbed NO, NO2, and NO3 on Pt(111), the most likely active metal face for this catalytic oxidation. NO, NO2, and NO3 are all strong electron acceptors and bind to the Pt(111) surface via charge donation from the surface. NO and NO2, in particular, exhibit a variety of adsorption geometries, the most favorable at low coverage being those that maximize surface−adsorbate charge transfer through binding to multiple surface Pt. At low coverage, the order of binding energies is NO > NO3> NO2, and the oxidation of adsorbed NO to NO2 is endothermic by 0.78 eV. Higher surface coverages favor migration of NO and NO2 to lower-coordination surface sites due to competition for metal d charge density. These changes ... read less

Abstract: A myriad of engineering applications involve contact between two surfaces, which induces localized plastic deformation near the surface asperities. As a generic problem in studying nanometer scale plastic deformation of solid surfaces, a unit process model of dislocation formation near a surface step under contact loading of a flat rigid surface is considered. The driving force on the dislocation is calculated using conservation integrals. The effect of surface adhesion, step size and lattice resistance on the dislocation driving force are analyzed in a continuum dislocation model, while the nucleation process is simulated atomistically. The driving force formula is used for a dislocation nucleation criterion and to get the equilibrium distance traveled by the dislocation away from the surface step. Results of the unit process model show that under a normal contact load dislocations nucleated in certain slip planes can only stay in a thin layer near the surface, while dislocations nucleated along other slip planes easily move away from the surface into the bulk material. The former dislocation is named anti-load dislocation and the latter dislocation is called pro-load dislocation. Embedded atom method (EAM) is utilized to perform the atomistic simulation of the unit-process model. As predicted by the continuum dislocation model, the atomistic simulations also indicate that surface adhesion plays significant role in dislocation nucleation process. Varying the surface adhesion leads to three different regimes of load-deflection instabilities, namely, just dislocation nucleation instability for no adhesive interaction, two distinct surface adhesion and dislocation nucleation instabilities for weak adhesive interaction and a simultaneous surface adhesion and dislocation nucleation instability for strong adhesive interaction. The atomistic simulations provide additional information on dislocation nucleation and growth near the surface steps. The results of dislocation segregation predict existence of a thin tensile-stress layer near the deformed surface and the results on the adhesion effect provides a cold-welding criterion. read less

Abstract: The deformation of copper with grain size less than 10 nm is investigated using a 2D continuum model incorporating atomistically-based constitutive relations. The local constitutive response of a series of symmetric and asymmetric tilt grain boundaries is obtained using an atomistic quasicontinuum method under tension and shear. The atomistic results show that it is possible to associate a constant maximum stress with each deformation mechanism triggered in the GB vicinity, i.e. GB sliding and decohesion, atom shuffling and partial dislocation emission. The GB strength is always found weaker in shear than in tension. This information is incorporated into a continuum polycrystalline model tested under compression. This model provides useful insights, in the absence of intragranular plasticity, into the onset of macroscopic quasi-plasticity, which results from GB sliding and collective grain rotation mechanisms. read less

Abstract: : Starting from two equilibrium solid solutions in the Au-Ni system, we analyze the change in composition due to a 400 eV/A fast ion track simulated by molecular dynamics in the Embedded Atom approximation. We aim at determining the influence of the thermodynamic forces derived from the large thermal gradients and the rapid solidification across the solidus and liquidus on the motion of solute atoms. One dimensional gradients as well as analytic models are used to quantitatively determine the domains of influence of these forces. Evidence shows that the liquidus and solidus equilibrium solidification predicted by the phase diagram is not reached during the track. The solute concentration is mainly determined by the combined diffusion and thermomigration mechanisms in the liquid stage. read less

Abstract: We investigate the role of electron–hole pair excitation in molecule–surface collisions by using a semiclassical model which incorporates coupling to phonons and electrons in the substrate. The model treats the dynamics of the incoming molecule by classical mechanics but quantizes the phonons and electrons using second quantization techniques. We find that neither phonons nor electron–hole pair excitation can be neglected for an accurate description of molecule–surface collisions. read less

Abstract: By introducing an expansion of the wavefunction for an N-atomic system in a Gauss–Hermite basis set is it possible to derive a trajectory based second quantization formulation of molecular dynamics. In the present paper we discuss some of the computational details of the theory and use it to treat an M=4 dimensional quantum mechanical system, namely that of hydrogen scattered from a solid surface. The full coupling to the surface phonons is also included in the calculations. read less

Abstract: We investigate structural and diffusion properties of CuN and PdN (with N going from 1 to 30) clusters adsorbed on the (110) surface of Pd via atomistic simulations performed by employing embedded atom method potentials. For both systems, one dimensional linear chains are lower in energy than two dimensional structures, although the linear chain stability is more enhanced in the case of Cu/Pd(110). Our results on cluster stability are analyzed in terms of effective interactions and adsorbate arrangement upon relaxation. In close connection with STM experiments performed recently on Cu/Pd(110) [Roeder et al., Nature 366, 141 (1993)] we evaluate the diffusion barrier for atomic movement along and across the [110] direction. A cross exchange mechanism is found to lower significantly the diffusion barrier across the [110] direction, consistent with the value of the diffusion anisotropy found experimentally and the phase separation observed at the uppermost layer level. read less

Abstract: Thermocapillary convection was investigated in a metallic system of pure Ni, at the nanoscale, by molecular dynamics. The system interface was irradiated by a heat flux, mimicking a focused laser source. The melt pool was submitted to a large temperature gradient that modified the surface tension along the interface. In liquid metal, because surface tension typically decreases with increasing temperature, the result is a gradient of surface tension along the free surface. The liquid metal, therefore, started to flow in the direction of high surface tension. Two counter-rotating convection cells developed, characteristic of those observed in welding and other material processing. A systematic estimation of relevant parameters in hydrodynamics allowed us to interpret the results in terms of Prandtl, Marangoni, and Péclet numbers. This study demonstrates the influence of laser power and system size on pool shape and flow characteristics. read less

Abstract: A series of molecular dynamics simulations were conducted to reveal the fatigue mechanisms in metal nanowires. We applied axial cyclic loading deformation on a copper single-crystal nanowire model and observed the deformation process during cycle evolution. The detailed observation revealed that the deformation mechanisms in the nanowire is essentially different from the case of the macro- and micro-scaled materials because of the lack of dislocation sources. We also found that atomic vacancies were formed continually by dislocation motion even under a simple single-slip condition. The accumulation of vacancies is expected to be a probable mechanism of fatigue in nanomaterials. read less

Abstract: Sputtered Cu/Si thin films were experimentally prepared at different sputtering pressures and characterized using X-ray diffraction (XRD) and an atomic force microscope (AFM). Simultaneously, an application-oriented simulation approach for magnetron sputtering deposition was proposed in this work. In this integrated multiscale simulation, the sputtered atom transport was modeled using the Monte Carlo (MC) and molecular dynamics (MD) coupling method, and the deposition of sputtered atoms was simulated using the MD method. This application-oriented simulation approach was used to simulate the growth of Cu/Si(100) thin films at different sputtering pressures. The experimental results unveiled that, as the sputtering pressure decreased from 2 to 0.15 Pa, the surface roughness of Cu thin films gradually decreased; (111)-oriented grains were dominant in Cu thin films and the crystal quality of the Cu thin film was gradually improved. The simulation results were consistent with the experimental characterization results. The simulation results revealed that the transformation of the film growth mode from the Volmer–Weber growth mode to the two-dimensional layered growth mode resulted in a decrease in the surface roughness of Cu thin films; the increase in the amorphous compound CuSix and the hcp copper silicide with the decrease in the sputtering pressure was responsible for the improvement of the crystal quality of the Cu thin film. This work proposed a more realistic, integrated simulation scheme for magnetron sputtering deposition, providing theoretical guidance for the efficient preparation of high-quality sputtered films. read less

Abstract: Under parallel electric fields and free evaporation conditions, the statics and dynamics of spreading–evaporating nanodroplets are investigated on an isothermally heated surface via molecular dynamics (MD) simulations. The simulation results show that at the substrate temperature of Ts = 320 K, the static and dynamic contact angles on the left and right edges are initially asymmetric and then symmetric with increasing field strengths of E = 0.00–0.06 V Å−1, resulting in the asymmetric-to-symmetric spreading transition of spreading–evaporating nanodroplets. Under weak evaporation condition, that is, at Ts = 320 K, the asymmetric-to-symmetric spreading transition is triggered by enhancing the intrinsic surface wettability θ0 = 49°–80° at a constant field strength of E = 0.03 V Å−1. However, at the substrate temperature of Ts = 350 K, the symmetric-to-asymmetric spreading transition first appears for the static and dynamic contact angles on the left and right edges, and then the asymmetric-to-symmetric spreading transition appears with increasing field strength. Under strong evaporation condition, that is, at Ts = 350 K, as the field strength is constant at E = 0.03 V Å−1, the asymmetric-to-symmetric spreading transition also appears with increasing surface wettability. read less

Abstract: In this paper, the bonding process of (100)- and (111)-oriented single crystal Cu bumps as well as (111)-oriented nanotwinned Cu (nt-Cu) bumps is studied intensively by molecular dynamics simulation. The influence of bonding temperature (473, 523 and 573 K) and bonding stress (0.5, 1 and 1.5 MPa) on the bonding process of the above three Cu bumps are characterized. It is found that the bonding degree of (111)-oriented single crystal Cu bumps is better than that of (100)-oriented ones, the shear strength of (111)-oriented Cu bump interconnects obtained under the condition of bonding temperature 473 K without bonding stress is about 22% higher than that of (100)-oriented ones. The bonding degree of (111)-oriented nt-Cu bump interconnects is slightly better than that of (111)-oriented ones, at bonding temperature 473 K and without bonding stress whose shear strength is about 18% higher than that of the latter. The use of (111)-oriented nt-Cu bumps can effectively improve the plasticity of Cu-Cu direct bonding interconnects. Moreover, the increase in bonding temperature can significantly improve the bonding degree of Cu bump interconnects, but the influence of bonding stress on the bonding degree of Cu bump interconnects is not completely positive. read less

Abstract: The structural and energetic properties of small silver clusters Agn with n = 2–100 atoms are reported. For n = 2–100 the embedded atom model for the calculation of the total energy of a given structure in combination with the basin-hopping search strategy for an unbiased structure optimization has been used to identify the energies and structures of the three energetically lowest-lying isomers. These optimized structures for n = 2–11 were subsequently studied further through density-functional-theory calculations. These calculations provide additional information on the electronic properties of the clusters that is lacking in the embedded-atom calculations. Thereby, also quantities related to the catalytic performance of the clusters are studied. The calculated properties in comparison to other available theoretical and experimental data show a good agreement. Previously unidentified magic (i.e., particularly stable) clusters have been found for n>80. In order to obtain a more detailed understanding of the structural properties of the clusters, various descriptors are used. Thereby, the silver clusters are compared to other noble metals and show some similarities to both copper and nickel systems, and also growth patterns have been identified. All vibrational frequencies of all the clusters have been calculated for the first time, and here we focus on the highest and lowest frequencies. Structural effects on the calculated frequencies were considered. read less

Abstract: Based on the non-equilibrium molecular dynamics simulation, a Cu/amorphous diamond/crystalline diamond sandwich model was established to investigate the effects of the amorphous degree diamond surface and the thickness of the amorphous layer on the thermal boundary conductance of Cu/crystalline diamond. The simulation results show that the thermal boundary conductance can be enhanced by diamond surface amorphization, and increases with the increase of the amorphous degree. For the fully amorphous layer, the thermal boundary conductance increases gradually with the increase of the thickness of the amorphous layer and can be enhanced up to 4 times. The analysis of the vibrational density of states, overlap energy and phonon participation ratio shows that the diamond surface amorphization promotes the vibrational coupling between diamond and Cu atoms at low frequencies, as well as the occurrence of phonon inelastic scattering, and thus improves the thermal transport capacity of interface. read less

Abstract: Molecular dynamics simulations are performed to reveal the underlying deformation mechanisms of gradient nano-grained materials with different-sized twins. The results indicate that the critical twin boundary spacing where the strength begins to soften decreases and the maximum strength of the material increases with the declining of the gradient. Below the critical value, the plastic deformation mechanism is dominated by the partial dislocations paralleling to the twin boundary, but when the twin boundary spacing exceed the critical value, the dislocation moved by the way of intersecting to the twin boundary. read less

Abstract: The functionalized applications of nanoporous metals place clear requirements on their basic mechanical properties, yet there is a lack of research on the mechanical response under multiaxial loading conditions. In this work, the mechanical behaviors of nanoporous gold under multiaxial tension are investigated via molecular dynamics simulations. The mechanical properties under different loading conditions are compared and the microstructure evolution is analyzed to clarify the deformation mechanisms of nanoporous gold in biaxial and triaxial tension. It is found that the modulus of nanoporous gold in multiaxial tension is strengthened and the strength is softened compared to uniaxial tension. The failure of nanoporous gold in multiaxial tension is dominated by the progressive yielding, necking, and rupture of ligaments along the multiple uniaxial loading directions. The dislocation activity under multiaxial loads is more intense and more prone to plastic deformation, ultimately resulting in lower strength and smaller failure strain. The findings provide more insight into the understanding of the deformation mechanisms of nanoporous metals under complex stress states. read less

Abstract: The application of Cu–Cu direct bonding in stacked chips is becoming an alternative to the traditional solder-bump joint during the development of three-dimensional integrated circuits (3D IC). In this paper, the self-sintering between copper nanoparticles (Cu NPs) and the sintering between copper nanoparticle (Cu NP) and copper substrate (metallization) are simulated by molecular dynamics method. The results show that Cu atoms move mainly by interdiffusion between adjacent Cu NPs during the early and middle stages of sintering process, and the self-sintering of Cu NPs occurs first. In the late stage of sintering process, Cu atoms begin to diffuse at the contact interface between Cu NP and Cu metallization, and Cu NPs are sintered with Cu metallization. In addition, the simulation results of the effects of sintering temperature and particle size of Cu NPs on the Cu–Cu sintering behavior manifest that a relatively higher sintering temperature and smaller particle size of Cu NPs can better promote the self-sintering between Cu NPs and the sintering between Cu NP and Cu metallization. It is also found that the suitable sintering temperature of Cu NP and Cu metallization is between 450 K and 600 K, and the particle size has little effect on the diffusion distribution of Cu atoms in Cu NPs and Cu metallization during the sintering process. read less

Abstract: In this study, the effect of adhesion on material removal is numerically investigated at the nanoscale level. In this regard, Molecular Dynamics (MD) simulations are conducted to distinguish the contribution of adhesive and abrasive wear mechanisms during a scratching process in terms of the degree of interfacial adhesion and scratching depth. The numerical model simulates the scratching of a flat workpiece made of a single crystalline aluminum using a rigid conical indenter with a blunted spherical tip at four different indentation depths (i.e. 0, 3, 7, and 10 Å). The classical Leonard-Jones interatomic potential is used to mimic the degree of adhesion by varying the adhesion parameter between 5–70% of the aluminum bonding energy. It is shown that, at shallow scratching depths, the contribution of adhesive work to the frictional work is much larger than the ploughing work. On the contrary, the ratio of adhesive to ploughing work reduces by increasing the scratching depth. read less

Abstract: Multisized nanoparticles (MPs) are widely employed as electronic materials to form conductive patterns, benefitting from their excellent sintering properties and mechanical reliability. However, due to the lack of effective detection methods for the real-time sintering process, it is difficult to reveal the sintering behavior during the MPs sintering process. In this work, a molecular dynamics method is used to track the trajectory of silver atoms. The melting behavior of a single nanoparticle (SP) is first discussed. The structural evolution of equally sized nanoparticles (EPs) and unequally sized nanoparticles (UPs) during the sintering process is analyzed alongside morphology changes. It is proposed that the UPs sintering process benefits from the wetting behavior of small-sized nanoparticles on the surface of large-sized nanoparticles, and the sintering angle (θ) is proposed as an index to estimate the sintering result of UPs. Based on the works above, three basic sintering modes and one advanced sintering mode in the MP sintering process are analyzed emphatically in this paper, and the roles of different-sized nanoparticles in MPs are concluded from simulation and experimental results. This work provides theoretical support for conductive ink composition design and sintering process optimization. read less

Abstract: As a representative of immiscible alloy systems, the Cu-Ta system was the research topic because of its potential application in industry, military and defense fields. In this study, an amorphous Cu-Ta alloy film was manufactured through magnetron sputter deposition, which was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Mechanical properties of Cu-Ta film were detected by the nanoindentation method, which show that the elastic modulus of Cu3.5Ta96.5 is 156.7 GPa, and the hardness is 14.4 GPa. The nanoindentation process was also simulated by molecular dynamic simulation to indicate the deformation mechanism during the load-unload stage. The simulation results show that the structure <0,2,8,4> and <0,2,8,5> Voronoi cells decreased by 0.1% at 50 Ps and then remained at this value during the nanoindentation process. In addition, the number of dislocations vary rapidly with the depth between indenter and surface. Based on the experimental and simulation results, the Voronoi structural changes and dislocation motions are the key reasons for the crystallization of amorphous alloys when loads are applied. read less

Abstract: The mechanical behaviors of uniaxial torsional and tensional copper nanorod embedded with sp2-type hybrid graphene nanosheets (3DG/Cu) were investigated systematically using molecular dynamics methods. During the torsion process, graphene expanded the plastic deformation region of copper, while the plastic deformation in monocrystalline Cu cases was limited to a smaller area. 3DG/Cu responded to the torsion by one more plastic stage when plastic deformation spread along the length after the elastic response. Graphene improved the torsional loading capacity of the composite material, greatly extending the effective response range of the material by distributing the deformation of copper along with the graphene rather than being concentrated at a certain position like monocrystalline Cu. Generally, as the length of the model increased, this enhancement decreased. The copper portion of 3DG/Cu was divided into three areas during uniaxial tensile, a static region, a quasi-static region of the middle portion where the shear and necking occurred, and a dynamic area near the loading end. However, the inside graphene kept continuous until fracture. Furthermore, graphene improved the yield strain of copper by maintaining intact after copper failure. The greater the pre-loaded torsion angle, the smaller the yield strength and Young’s modulus of 3DG/Cu. read less

Abstract: Neutron radiation induces point defects and affects the diffusivity of atoms and the kinetics of precipitation. The phase-field simulation reveals the influence of migration energy of vacancy on the radiation-enhanced precipitation in Fe–Cu alloy. The study shows that radiation-enhanced diffusion (RED) also depends on the diffusivity of vacancy-associated migration energy and not only on the dose rate; the low migration energy of vacancy results in accelerated precipitation and a higher volume fraction of Cu precipitates. Interestingly, decreasing migration energy from 1.0 eV to 0.9 eV results in a 30% increase in the precipitates’ volume fraction. Also, the combination of the lowest dose rate 5.0 × 10−3 dpa s−1 and highest migration energy 1.0 eV delays the precipitation. The study also examines the influence of migration energy of vacancy on the radius of Cu precipitates. The lowest migration energy, 0.9 eV, increases the radius up to one-third. Finally, the work presents the drawbacks of the analytical digital image processing technique in the quantitative comparison with the script. read less

Abstract: Volumetric defects in crystals worsen operational properties of structural materials; therefore, the problem of reducing discontinuities in solid is one of the most important in modern materials science. In the present work, the results of computer simulation are presented that demonstrate possibility of collapse of pores in a crystal in state of shear deformation under the influence of shock waves. Similar waves can occur in a solid under external high-intensity exposure. For example, in the zone of propagation of displacement cascade, there are regions in which occurs a mismatch between the thermalization times of atomic vibrations and the removal of heat from them. As a result of the expansion of such a region, a shock after cascade wave arises. The simulation was carried out based on molecular dynamics method using the potential calculated by means of mmersed atom method. As a bulk defect, we considered extended pores of cylindrical shape, which can be formed after passing of high-energy ions through a crystal, or, for example, when superheated closed fluid inclusions (mother liquor) reach the surface. The study has shown that such defects are the source of heterogeneous nucleation of dislocation loops, contributing to a decrease in the shear stresses in simulated structure. Dependences of the average dislocation density on the shear angle and temperature of the designed cell were established, and the loop growth rate was estimated. Generated shock waves create additional tangential stresses that contribute to the formation of dislocation loops; therefore, in this case, dislocations are observed even with a small shear strain. If during simulation the thermal effect increases, the pore collapses. read less

Abstract: Single-crystal copper (Cu), whose atom arrangement is in the same direction and has no grain boundary, is widely used in defense technology, civil electronics and network communication. As a diamond turnable material, fan-shaped patterns appear on the machined surface, which affects the machined surface quality and the optical function it carries. Previous studies on the surface generation mechanism in single-point diamond turning (SPDT) of Cu were limited to experimental analysis, while there is a lack of fundamental understanding of the fan-shaped pattern generation mechanism and suppression method. In the present study, the different fan-shaped patterns, surface quality, cutting force and chip morphology of the typical crystal planes (100), (110) and (111) planes of Cu were studied by both theoretical and experimental analyses. A molecular dynamics (MD) simulation was conducted to present the fundamental generation mechanism of the fan-shaped patterns from atom arrangement directions and its angle change with the main cutting direction, while a cutting dynamics model was established to simulate the generation of fan-shaped patterns on the machined surface. Based on theoretical and experimental analysis, it was found that the atom density arrangement directions of Cu and its angle change with the main cutting direction of SPDT caused fluctuations in the friction coefficient, which further caused the vibration of the cutting system and generated the fan-shaped patterns. The SPDT of crystal planes (100) can achieve the best surface quality. The present research provides a fundamental understanding of fan-shaped pattern formation on the machined surface, and provides an instruction for machining Cu to obtain better surface quality. read less

Abstract: With the popularization of 3D printing technology, micro/nanoparticles sintering technology has drawn lots of attentions all over the world. Here, molecular dynamic simulation is employed to discuss the effects of different interfacial lattice structures, different diameter of nanoparticles, and different heating rates on the coalescence of metallic Cu nanoparticles. The results showed that the diameter of nanoparticles determine the melting point of the system. Besides, the interfacial lattice structure, diameter of nanoparticles, and heating rate have an influence on the initial sintering temperature. This is because the melting point is the inherent property of material which relies on the mass of substance. However, the initial sintering temperature is sensitive to many factors, including the temperature, interfacial, and intermolecular interactions. read less

Abstract: In this work, we will shed light on the results obtained from the molecular dynamics method in the temperature ranging of 300-700K. This investigation concerning the coalescence for the two tetramers islands of system having different forms (SS, TT and NN) deposited at different spacing sites in-channel and cross-channel. The stability of the systems, the diffusion phenomena produced during the dynamics, as well as the diffusion/coalescence lifetime during the evolution of temperature are dependent. The dynamic study shows that between 450K and 650K the homogeneous partial coalescence occurs through the mechanism of the jump. As time goes by increasing the temperature which favors the process of exchange the heterogeneous total coalescence is obtained. read less

Abstract: Extremely small cutting depths in nanoscale cutting makes it very difficult to measure the thermodynamic properties and understand the underlying mechanism and behavior of workpiece material. Highly precise single-crystal Cu is popularly employed in optical and electronics industries. This study, therefore, implements the molecular dynamics technique to analyze the cutting behavior and surface and subsurface phenomenon in the nanoscale cutting of copper workpieces with a diamond tool. Molecular dynamics simulation is carried out for different ratios of uncut chip thickness (a) to cutting edge radius (r) to investigate material removal mechanism, cutting forces, surface and subsurface defects, material removal rate (MRR), and stresses involved during the nanoscale cutting process. Calculation of forces and amount of plowing indicate that a/r = 0.5 is the critical ratio for which the average values of both increase to maximum. Material deformation mechanism changes from shear slip to shear zone deformation and then to plowing and elastic rubbing as the cutting depth/uncut chip thickness is reduced. The deformation during nano-cutting in terms of dislocation density changes with respect to cutting time. During the cutting process, it is observed that various subsurface defects like point defects, dislocations and dislocation loops, stacking faults, and stair-rod dislocation take place. read less

Abstract: In this work, molecular dynamics simulations have been used to simulate the behavior of tetramer clusters behavior in Pt4 /Cu (110) and Au4 /Ag (110) systems, in the temperature range 300-600 K. All activation barriers and formation energies related to different tetramer shapes (4S, 4L, 4T, 4N and 4l) have been calculated by embedded atom method (EAM) at static regime (0 K). From an energetical point of view, the adatoms tend to diffuse via simple jumps and exchange mechanisms leading to a transition between all forms during tetramer diffusion. Statistical analysis after molecular dynamics simulations confirms that the linear 4l shape is more stable and needs high energy to be disintegrated in both systems. The lifetime study of each shape for different temperatures (from 300 K to 600 K) proves that the 4 l form is more stiff, which is in a good agreement with the formation energy predictions. read less

Abstract: The behavior of Ag/Au/Cu nanoparticles dispersed on Si substrates during heat treatment was studied by molecular dynamics in this paper. Whether for the thermal welding of homogeneous metal nanoparticles or heterogeneous metal nanoparticles, it was found that the fundamental reason for the contact between the surface of metal nanoparticles is the electron exchange, which reduces the energy of the system and leads to welding contact, then the atoms migrate toward the interior of nanoparticles through nucleation and growth process, resulting in the decrease of the porosity and the shrinkage of the sample. At the same time, the simulation results show that when the applied temperature rises to a certain extent, the adjacent nanoparticles may condense into isolated structures under their own surface tension and stress, as well as the adhesion of the substrate, which will make the conductive structure discontinuous. However, by adjusting the size and proportion of Ag/Au/Cu metal nanoparticles, besides saving the cost of printing circuits, the conductive structure can be more continuous and the conductivity can be enhanced. read less

Abstract: Correction for 'The effect of pulse duration on nanoparticle generation in pulsed laser ablation in liquids: insights from large-scale atomistic simulations' by Cheng-Yu Shih et al., Phys. Chem. Chem. Phys., 2020, 22, 7077-7099, DOI: 10.1039/d0cp00608d. read less

Abstract: Aluminum alloys development always exit in the manufacturing process. Al/Mg alloys have been attracted significant attention because of their excellent mechanical properties. The microstructural evolution and deformation mechanisms are still challenging issues, and it is hard to observe directly by experimental methods. Accordingly, in this paper atomic simulations are performed to investigate the uniaxial compressive behavior of Al/Mg phases; with different ratio of Mg ranging from 31% to 56%. The compression is at the same strain rate (3.1010 s⁻¹), at the same temperature (300K) and pressure, using embedded atom method (EAM) potential to model the interactions and the deformation behavior between Al and Mg.From these simulations, we get the radial distribution function; the stress–strain responses to describe the elastic and plastic behaviors of β-Al3Mg2, ε-Al30Mg23, Al1Mg1 and γ-Al12Mg17 phases with 31, 41, 50 and 56% of Mg added to pure aluminum, respectively. The mechanical properties, such as Young’s modulus, elasticity limit and rupture pressure, are determined and presented. The engineering equation was used to plot the stress-strain curve for each phase.From the results obtained, the chemical composition has a significant effect on the properties of these phases. The stress-strain behavior comprised elastic, yield, strain softening and strain hardening regions that were qualitatively in agreement with previous simulations and experimental results. These stress-strain diagrams obtained show a rapid increase in stress up to a maximum followed by a gradual drop when the specimen fails by ductile fracture. Under compression, the deformation behavior of β-Al3Mg2 and γ-Al12Mg17 phases is slightly similar. From the results, it was found that ε-Al30Mg23 phase are brittle under uniaxial compressive loading and γ-Al12Mg17 phase is very ductile under the same compressive loading.The engineering stress-strain relationship suggests that β-Al3Mg2 and γ-Al12Mg17 phases have high elasticity limit, ability to resist deformation and also have the advantage of being highly malleable. From this simulation, we also find that the mechanical properties under compressive load of ε-Al30Mg23 phase are evidently less than other phases, which makes it the weakest phase. The obtained results were compared with the previous experimental studies, and generally, there is a good correlation.The Al-Mg system was built and simulated using molecular dynamics (MD) software LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). read less

Abstract: We study the size dependencies on melting of Gold nanoparticle. The nanoparticles are built with different sizes and heated up with same temperature gradient from room temperature up to 1400 K. The trajectories of the atoms are investigated based on Molecular Dynamics (MD) simulation. Pressure evolution as a function of time shows the oscillation pattern as in the case of thermal induced melting. Analysis based on structure factor combined with Common Neighbour Analysis (CNA) indicated the properties of melting depends on nanoparticle sizes. read less

Abstract: This paper aims to investigate atoms type and channel roughness effects on fluid behavior in nanochannel.,The results of mechanical properties of these structures are reported in this work by using molecular dynamics method.,The results show that nanochannel roughness is a limiting factor in flowing fluid in nanochannel. Moreover, fluids with less atomic weight have more free movement in ideal and non-ideal nanochannels.,For the study of mechanical properties of fluid/nanochannel system, the authors calculated parameters such as potential energy, density, temperature and velocity profiles of simulated fluids. read less

Abstract: Morphological and Structural transitions during epitaxial growth of surface and interface are an important nanoscal phenomena. Understanding the diffusion of adatom in anisotropic surface on the fc... read less

Abstract: Anisotropy is one central influencing factor on achievable ultimate machined surface integrity of metallic materials. Specifically, grain boundary has a strong impact on the deformation behaviour of polycrystalline materials and correlated material removal at the microscale. In the present work, we perform molecular dynamics simulations and experiments to elucidate the underlying grain boundary-associated mechanisms and their correlations with machining results of a bi-crystal Cu under nanocutting using a Berkovich tool. Specifically, crystallographic orientations of simulated bi-crystal Cu with a misorientation angle of 44.1° are derived from electron backscatter diffraction characterization of utilized polycrystalline copper specimen. Simulation results reveal that blocking of dislocation motion at grain boundaries, absorption of dislocations by grain boundaries and dislocation nucleation from grain boundaries are operating deformation modes in nanocutting of the bi-crystal Cu. Furthermore, heterogeneous grain boundary-associated mechanisms in neighbouring grains lead to strong anisotropic machining behaviour in the vicinity of the grain boundary. Simulated machined surface morphology and machining force evolution in the vicinity of grain boundary qualitatively agree well with experimental results. It is also found that the geometry of Berkovich tool has a strong impact on grain boundary-associated mechanisms and resultant ploughing-induced surface pile-up phenomenon. read less

Abstract: A series of dynamic shock compressions of metal films are simulated using molecular dynamics method. Characteristics of mechanical shock load are analyzed theoretically. Thermomechanical responses induced by mechanical shock, including pressure, temperature, volumetric strain, and shear strain, are presented graphically and analyzed from viewpoint of thermomechanical coupling. The results show that the temperature rise caused by volumetric strain (elastic) is reversible, while that caused by shear strain is irreversible. Under the same mechanical shock load, several metal films exhibit significant differences in thermomechanical responses and the potential reasons are analyzed from the differences of thermophysical parameters. read less

Abstract: During nanofabrication, the processing of different crystal faces along the surface of a workpiece varies because of size effects. The burr height and subsurface damage during nanogrinding of different crystal orientations in monocrystalline nickel are simulated by molecular dynamics in this study. The burr height and depth of the subsurface deformation layer are calculated quantitatively by atomic displacement, common neighbor analysis and dislocation analysis techniques. Among the directions considered in this paper, the burr height is the smallest when grinding in the (110)[110] direction, and the depth of the subsurface deformation layer is small when grinding along the (111) plane. This study demonstrates the effect of different slip paths on burr height and provides a reference for the actual processing of face-centered cubic crystals. read less

Abstract: The reinforcement of various materials by nanofillers as nanocomposites has recently received the attention of many researchers. In the present research, molecular dynamics simulations are used to investigate the influence of nanowire (NW)/carbon nanotube (CNT) reinforcement on the buckling behavior of metallic glass matrix nanocomposites (MGMNCs). The buckling characteristics of nanocomposites made by adding Cu NWs, CNTs and Cu NW-encapsulated CNTs to metallic glass matrices are studied. The results demonstrate that MG alloys comprising just two elements (Cu and Zr) with higher Cu percentage have higher mechanical stability. Also, it is observed that adding NW leads to a negative effect on the buckling behavior, while adding CNT and NW-encapsulated CNT considerably increases the buckling force and strain of the metallic glass models. Moreover, it is found that the filled CNT is the most effective nanofiller for amending the buckling behavior of metallic glasses. Furthermore, as the size of nanofillers gets larger, the critical force increases and the critical strain decreases. read less

Abstract: Polycrystalline materials can be divided into four types of microstructural components, including grain cell (GC), grain boundary (GB), triple junction (TJ) and vertex points (VP). Nanoindentation at different microstructural components on the polycrystalline materials surface can lead to different plastic deformation behaviors of the polycrystalline materials. Due to experimental limitations, the indentation-induced internal stress and defect evolution process are difficult to investigate directly, especially for the polycrystalline materials with grain size less than 100[Formula: see text]nm. The molecular dynamics (MD) simulations were performed to unravel the initial indentation position effect on the elasticity/plastic deformation mechanism of polycrystalline copper. The results reveal that the initial indentation position governs the indentation force variation and defect distribution range due to the different dimensionalities of the microstructural components. The defect propagation as well as the internal stress transmission in the GC regions tend to transfer to the low-dimensional microstructural components of the interfaces. In addition, the atomic internal stress and potential energy accumulation/release of the microstructural component atoms during the nanoindentation process are also investigated, revealing that the atomic internal stress and potential energy in the VPs vary earliest, followed by the TJs, GBs and GCs. read less

Abstract: The endohedral functionalization of single-walled carbon nanotubes with molecular species, nanowires (NWs) and nanoparticles is of great importance for fabrication and development of nanoelecronic devices, drug delivery and energy storage applications. This research intends to explore the axial buckling behavior of the endohedrally functionalized single-walled carbon nanotubes (SWCNTs) by various metallic NWs (mNW@SWCNT), i.e. aluminum, copper, iron, sodium, nickel (AlNW, CuNW, FeNW, NaNW, NiNW), considering all possible pentagonal configurations. Employing the molecular dynamics (MD) simulations, the results demonstrate that the structurally stable radius of SWCNTs for successful endohedral functionalization of SWCNTs with pentagonal NWs are different. Considering buckling analysis of models, it is observed that NWs, solely, cannot tolerate any axial compressive load and their structure becomes dramatically unstable under mechanical force. By inserting NWs inside SWCNTs, their pentagonal structures during simulation are preserved due to Vdw interaction of NW and SWCNT until buckling occurs. Moreover, the buckling simulation results indicate that by increasing the length, the critical force of mNW@SWCNT decreases and approximately tends to that of pure SWCNTs which is more considerable for AlNWs. Also, in the particular length, the encapsulation of NWs inside the SWCNTs causes a considerable increase in the critical buckling forces particularly in smaller lengths. According to the attained results, functionalization of SWCNTs with E and S configuration of AlNWs improves the structural stability of SWCNTs more pronounced than other pentagonal NWs. read less

Abstract: The mechanical properties of a material can be positively or negatively affected by its applied or residual stress. In this article, a series of molecular dynamic simulations were adopted to invest... read less

Abstract: Grain boundaries (GBs) in bulk nanostructured materials processed by severe plastic deformation (SPD) have a nonequilibrium structure caused by extrinsic grain boundary dislocations (EGBDs) absorbed during deformation. Under external influences (annealing or cyclic straining) these GBs relax towards equilibrium resulting in a release of the excess energy of nanomaterials. Using our earlier developed method, atomic models of nanocrystals with nonequilibrium GBs having grain sizes of 10, 15, and 20 nm are constructed and relaxed by molecular dynamics. Then, the effect of oscillating tension-compression stresses with amplitudes of 2, 3 and 4 GPa on the systems thus obtained is simulated. Relaxation of nonequilibrium GBs by dislocation emission is found and the effect is shown to be independent of the grain size. read less

Abstract: The phase transition characteristics of AgxCu(500−x) (x = 0, 100, 125, 200, 250, 300, 375, 400, 500) alloy particles are studied by molecular dynamics using the embedded atom potentials. We find that the Ag-Cu alloy particles tend to be Cu-core/Ag-shell structure. Ag300Cu200 alloy particle forms a glass phase at a quenching rate of more than 2 × 1011 K s−1. It is an atom-level amorphous alloying particle. The binary Ag-Cu alloy particle with stoichiometry Ag3Cu2 is favorable for producing amorphous phase and the phase transition temperature of Ag-Cu alloy particles is typically less than or close to the phase transition temperature of the Ag particle at the same size. read less

Abstract: In our present study, under uniaxial tension, atomistic simulations were conducted to explore the crack propagation mechanism of Square Nickel Plate (SNP) for two distinct shaped cracks (Rectangular and Circular) at center separately. Here, for modeling the inter-atomic potential between atoms, Embedded Atom Model (EAM) was used. In case of both types, the crack size was varied keeping a constant strain rate of 2×109 s-1 and temperature of 300 k for investigation of the effects of crack geometry and size on the behavior of crack propagation. Along with the size and geometry of crack, the effects of different strain rates (1×109, 2×109 and 4×109 s-1) and temperatures (300 K, 600 K and 900 k) were also studied. From the simulations, the declination nature of peak stress can be deduced for both of the geometries by increasing the crack size. It can also be concluded that when crack area was same, the peak stresses were higher in SNP with Circular crack than with the SNP with Rectangular one. Besides, increasing and decreasing nature of peak stress were found for two genres with the increment of strain rate and temperature separately. read less

Abstract: Atomistic simulations of the structure, energy and relaxation under the action of high frequency cyclic straining are carried out for columnar nickel nanocrystals with [112] column axis, the grain boundaries (GBs) of which are in a nonequilibrium state caused by the presence of extrinsic grain boundary dislocations (EGBDs). A special method of introducing EGBDs is used to create initial structures with nonequilibrium GBs. Energy of GBs as a function of the degree of nonequilibrium is evaluated and qualitatively compared to the results of dislocation and disclination modeling. It is shown that under loading by symmetrically oscillating stresses the nonequilibrium GBs generate lattice dislocations, which travel across the grains and are absorbed by opposite GBs thus resulting in a relaxation of the structure, long-range stress fields and the energy of GBs. read less

Abstract: Evolution of deformation and stress in growing thin films has been studied in this work using computational simulations that resolve matter at atomic length and time scales. For the surface layers of films laying on the substrate of a dissimilar material, the stress distribution analysis around defects becomes more challenging. Herein, spatial and temporal distribution of deformation and associated stress evolution are presented for different thin film formation events including (1) sub-monolayer growth during an early film nucleation stage and (2) coalescence of adjacent monolayer “islands.” Validity of the stress computed via local computations of the virial expression for stress in a system of interacting particles was checked by comparing to results obtained from considerations of local atomic deformation in conjunction with existing expressions for epitaxial thin film growth stress. For the geometries studied here, where a monolayer of film with a highly characterized linear defect, as in the case of a stacking fault, was simulated for coalescence, fairly good agreement was found. This result demonstrates that, for similar defects at the surface layer, with sufficient sub-ensemble averaging of the standard virial expression for stress, semiquantitative spatial stress distribution information can be obtained from atomic scale simulations. Using our validated stress computation method, we reveal significant stress localization during thin film growth processes, leading to pronounced differences in maximum and minimum stress observed over very small spatial extent (of order multiple GPa over 3–6 nm distances). One prominent mechanism of stress localization revealed here is coalescence between adjacent growing islands. For geometries explored here, stress manifesting during coalescence is highly localized. read less

Abstract: Hollow nanoparticles (hNPs) are of interest because their large cavities and small thickness give rise to a large surface to volume ratio. However, in general they are not in equilibrium and far from their global energy minimum, which often makes them unstable against perturbations. In fact, a temperature increase can induce a structural collapse into a nanoparticle, and consequently a loss of their unique properties. This problem has been studied by means of molecular dynamics (MD) simulations, but without emphasis on the speed of the temperature increase. Here we explore how the temperature variation, and the rate at which it is varied in MD simulations, determines the final conformation of the hNPs. In particular, we show how different temperature ramps determine the final shape of Pt hNPs that initially have an external radius between 0.7 and 24 nm, and an internal radius between 0.19 and 2.4 nm. In addition, we also perform the simulations of other similar metals like Ag and Au. Our results indicate that the temperature ramp strongly modifies the final hNP shape, even at ambient temperature. In fact, a rapid temperature increase leads to the formation of stacking faults and twin boundaries which are not generated by a slower temperature increase. Quantitative criteria are established and they indicate that the stacking fault energy is the dominant parameter. read less

Abstract: Nanoscratching and nanoindentation simulations are performed to study the processability of Cu/Ni bilayers with interfaces using molecular dynamics (MD) method. Single crystals Cu and Ni are served as comparisons. In the nanoscratching processes, the interfaces of Cu/Ni bilayers appear as a barrier of dislocations gliding, and lead to larger friction forces and normal forces. For single crystals and bilayers, both their friction forces and normal forces increase with the increasement of scratch velocity at 100-300 m/s. Friction coefficients under scratching processes are calculated, and they are smaller than macrosacle scratching process because of coating effects of nano-chips on the tool. The effects are analyzed by conducting both molecular dynamics simulations in nanoscale and finite element simulations (FES) in macroscale. In the indentation process, the processing properties of Cu-Ni and Ni-Cu bilayers are different from each other, and their indentation forces are both larger than their single crystals. Recovery deformation takes place during the relaxation stage. When the tool is unloading, some workpiece atoms adhere to the tool. The simulation results of the two nanoscale machining processes reveal the strengthening mechanism of interface, and show comprehensive processability of metal bilayers. read less

Abstract: Atomic structure of nonequilibrium [112] tilt grain boundaries in nickel containing disclination dipoles is studied by means of molecular dynamics simulations. Initial systems for simulations are constructed by joining together pieces of two bicrystals one of which contains a symmetric tilt GB S=11 / 62.96° and the other a GB S=105 / 57.12°, or S=125 / 55.39°, or S=31 / 52.20°, so disclination dipoles with strengths w = 5.84°, 7.58° and 10.76° are created. Stress maps plotted after relaxation at zero temperature indicate the presence of high long-range stresses induced by disclination dipoles. Excess energy of GBs due to the nonequilibrium structure is calculated. Effect of oscillating tension-compression stresses on the nonequilibrium GB structure is studied at temperature T = 300 K. The simulations show that the oscillating stress results in a generation of partial lattice dislocations by the GB, their glide across grains and sink at appropriate surfaces that results in a compensation of the disclination stress fields and recovery of an equilibrium GB structure and energy. read less

Abstract: in the investigation of the fundamental mechanisms of lasermaterial interactions. The advancements in the computational methodology and fast growth of available computing resources are rapidly expanding the range of problems amenable to atomistic modeling. This chapter provides an overview of the results obtained in recent simulations of laser ablation of metal targets in vacuum, a background gas, and a liquid environment. A comparison of the Chapter 12 read less

Abstract: We use molecular dynamics simulations to study the mechanical properties of a columnar nanocrystalline copper with a mean grain size between 8.91 nm and 24 nm. The used samples were generated by using a melting cooling method. These samples were submitted to uniaxial tensile test. The results reveal the presence of a critical mean grain size between 16 and 20 nm, where there is an inversion in the conventional Hall-Petch tendency. This inversion is illustrated by the increase of flow stress with the increase of the mean grain size. This transition is caused by shifting of the deformation mechanism from dislocations to a combination of grain boundaries sliding and dislocations. Moreover, the effect of temperature on the mechanical properties of nanocrystalline copper has been investigated. The results show a decrease of the flow stress and Young’s modulus when the temperature increases. read less

Abstract: The validation of classical potentials for describing multicomponent materials in complex geometries and their high temperature structural modifications (disordering and melting) requires to verify both a faithful description of the individual phases and a convincing scheme for the mixed interactions, like it is the case of the embedded atom scheme. The present paper addresses the former task for an embedded atom potential for copper, namely the widely adopted parametrization by Zhou, through application to bulk, surface and nanocluster systems. It is found that the melting point is underestimated by 200 degrees with respect to experiment, but structural and temperature-dependent properties are otherwise faithfully reproduced. read less

Abstract: Molecular dynamics (MDs) simulations were used to explore the thermal stability of Au nanoparticles (NPs) with decahedral, cuboctahedral, icosahedral and Marks NPs. According to the calculated cohesive energy and melting temperature, the Marks NPs have a higher cohesive energy and melting temperature compared to these other shapes. The Lindemann index, radial distribution function, deformation parameters, mean square displacement and self-diffusivity have been used to characterize the structure variation during heating. This work may inspire researchers to prepare Marks NPs and apply them in different fields. read less

Abstract: Nanoindentation is a useful experimental method to characterize the micromechanical properties of materials. In this study molecular dynamics and peridynamics are used to simulate nanoindentation, with a spherical indenter targeting a thin single crystal Cu film, resting on an infinitely stiff substrate. The objective is to compare the results obtained from molecular dynamic simulations to those obtained using a peridynamic approach as regards the force-displacement curves and the deformation patterns after that the material parameters in the peridynamic model have been fitted to the force displacement curve from the molecular dynamic simulation. read less

Abstract: In this work we report on terahertz phononic excitations in two-dimensional (2D) gold nanoparticle arrays in a water matrix through a series of large-scale molecular dynamics simulations. For the first time, we observe acoustic Dirac-like crossings in H (H2O) atomic (molecular) networks that emerge due to an intraband phononic scattering. These crossings are the phononic fingerprints of ice-like arrangements of H (H2O) atomic (molecular) networks at nanometer scale. We reveal how phononic excitations in metallic nanoparticles and the water matrix reciprocally impact on one another providing the mechanism for the THz phononics manipulation via structural engineering. Furthermore, we show that by tuning the arrangement of 2D gold nanoparticle assemblies, the Au phononic polarizations experience subterahertz hybridization (Kohn anomaly) due to surface electron–phonon relaxation processes. This opens the way for the sound control and manipulation in soft matter metamaterials at nanoscale. read less

Abstract: The structure of amorphous materials near the interface with an ordered substrate can be affected by various characteristics of the adjoining phases, such as the lattice spacing of the adherent surface, polymer chain length, and adhesive strength. To discern the influence of each of these factors, four FCC metal lattices are examined for three chain lengths of n-alkane and van der Waals interfacial interactions are controlled by adjusting the Lennard-Jones 12-6 potential parameters. The role of interaction strength is investigated for a single chain length and substrate combination. Four nanoconfined systems are also analyzed in terms of their mechanical strength. A strong layering effect is observed near the interface for all systems. The distinctiveness of polymer layering, i.e., the maximum density and spatial extent, exhibits a logarithmic dependence on the interaction strength between polymer and substrate. Congruency with the substrate lattice parameter further enhances this effect. Moreover, the elastic modulus of the alkane phase as a function of layer thickness indicates that the effects of ordering within the structure extend beyond the immediately obvious interfacial region. read less

Abstract: A molecular dynamics study of edge cracks in Ni and Al single crystals under mode-I loading conditions is presented. Simulations are performed using embedded-atom method potentials for Ni and Al at a temperature of 0.5K. The results reveal that Ni and Al show different fracture mechanisms. Overall failure behavior of Ni is brittle, while fracture in Al proceeds through void nucleation and coalescence with a zig-zag pattern of crack growth. The qualitative nature of results is discussed in the context of vacancy-formation energies and surface energies of the two FCC metals. read less

Abstract: Structures of simulated metals (Al, Cu, Ni) obtained in result of isothermal annealing after quick cooling to certain temperatures are studied in detail. Obtained large enough structures heave a core skeleton from hcp-planes in the form of icosahedron that already are named “Ih-fractal” (tetrahedra with internal fcc structure are divided one from another by twinning hcp-planes). Rows from elementary decahedra in structures of hcp-planes give the basis of new 60 twinning planes for forming new sectors at growing of such formation. New rows from decahedra are appearing in the places of intersections of the secondary twinning planes, and a new family of twinning planes is forming. read less

Abstract: In this paper, we investigate the dynamic shear strength of perfect monocrystalline metals using the molecular dynamics simulation. Three types of deformation (single shear, uniaxial compression and tension) are investigated for five metals of different crystallographic systems (fcc, bcc and hcp). A strong dependence of the calculated shear strength on the deformation type is observed. In the case of bcc (iron) and hcp (titanium) metals, the maximal shear strength is achieved at the uniaxial compression, while the minimal shear strength is observed at the uniaxial tension. In the case of fcc metals (aluminum, copper, nickel) the largest strength is achieved at the pure shear, the lowest strength is obtained at the uniaxial compression. read less

Abstract: In the present work we investigate the nanometric cutting of a nanotwinned Cu containing 26° inclined twin boundaries using a diamond cutting tool by means of molecular dynamics simulations, with a focus on examining the influence of rake angle of cutting tools on the cutting processes. The underlying deformation mechanisms of the material are elucidated and are further correlated with the evolution of machining forces and the formation of machined surface and chips. Our simulation results indicate that dislocation slip, interaction of dislocation with twin boundaries and twin boundaries-associated mechanisms work in parallel in the plastic deformation of the nanotwinned Cu. It is found that the rake angle has a significant influence on the deformation behaviour of the material, chip formation and machined surface quality. A rake angle of 45° results in smaller energy dissipation and better machined surface quality than the other two rake angles of 0° and −45°. read less

Abstract: The structure of overheated metal clusters appeared in condensation process was studied by computer simulation techniques. It was found that clusters with size larger than several tens of atoms have three layers: core part, intermediate dense packing layer and a gas- like shell with low density. The change of the size and structure of these layers with the variation of internal energy and the size of cluster is discussed. read less

Abstract: We carry out both four-dimensional (4D×2D) and six-dimensional (6D) quantum dynamics on a parametrically time- and temperature-dependent effective Hamiltonian for H2/D2(v = 0,j = 0)–Ni(100) collision process. Such an effective potential was derived within a theoretical framework of mean-field approximation by considering weakly correlated interaction between molecular degrees of freedom, phonon modes and electron– hole pair (elhp) coupling through a Hartree-product-type wave function, where the initial state distribution of the surface modes and elhp coupling were introduced through Bose– Einstein and Fermi– Dirac probability factor, respectively. The temperature-dependent dissociation and state-to-state transition probabilities for H2/D2(v = 0,j = 0)–Ni(100) system are depicted as a function of initial kinetic energ of the incoming diatom. Though such effect appears negligibly small for H2(v = 0,j = 0)–Ni(100) system, it is prominent in the case of D2(v = 0,j = 0)–Ni(100) collision. It appears that the change of dissociation and transition probabilities of D2 with the increase of surface temperature is exclusively dictated by the phonon modes directed along Z-axis, but the effect of elhp coupling particularly for transition probabilities is insignificant. read less

Abstract: For ductile metals, the process of dynamic fracture occurs through nucleation, growth and coalescence of voids. For high purity single-phase metals, it has been observed by numerous investigators that voids tend to heterogeneously nucleate at grain boundaries and all grain boundaries are not equally susceptible to void nucleation. However, for materials of engineering significance, especially those with second phase particles, it is less clear if the type of bi-metal interface between the two phases will affect void nucleation and growth. To approach this problem in a systematic manner two bi-metal interfaces between Cu and Pb have been investigated: {111} and {100}. Qualitative and quantitative analysis of the collected data from molecular dynamics shock and spall simulations suggests that Pb becomes disordered during shock compression and is the preferred location for void nucleation under tension. Despite the interfaces being aligned with the spall plane (by design), they are not the preferred location... read less

Abstract: The development of new materials impacts on all branches of engineering and, in particular, in aerospace engineering. Metallic glasses (MG) are relatively newcomers to the world of materials science and have excellent mechanical properties; its study is mandatory to allow its technological implantation. The macroscopic mechanical properties of a material are linked to its atomic structure. In particular, the fracture behaviour of brittle materials is initiated by the generation of vibrational modes. In metallic glasses, with amorphous structure, the vibrational spectrum has specific features. In this work, the vibrational properties of metallic glasses are examined by Molecular Dynamics simulations. The study was focused in binary systems, which were simulated using different interatomic potentials: Lennard-Jones (LJ), Morse and the semiempirical Embedded atom (EAM). As in Pd-based metallic glasses, the ratio of masses of both species was high, namely 2 in Lennard-Jones potentials and 1.67 in Morse and EAM potentials. The large scale simulations allowed us to simulate systems with nm-scale heterogeneities frozen-in during the quenching process. Different relaxation states were obtained by changing the quenching rates of the simulated MGs. The collective vibrational atomic dynamics of metallic glasses is a longstanding subject of debate. The origin of the excess of vibrational modes known as Boson Peak (BP) is not clear. In the systems analysed the dependence of the BP position and intensity on the system size is found to be weak. On the contrary, the BP intensity increases with the quenching rate, while its position shifts slightly to smaller frequencies. The results obtained by using realistic, semiempirical EAM potentials compare well with the experimental data available in glasses of similar compositions. The dynamic structure factor, S(q, ¿) is also computed in large systems to get information on the behaviour of acoustic excitations at low wavenumbers. The dominant frequencies O L,T (q) are determined for each considered wavevector, in order to compute the relation of dispersion of longitudinal and transverse phonons. In all studied cases the width of the peak, GL,T(q), increases as the frequency increases. A linear region at low wavenumbers and a bending when approaching the limit of the first pseudo-Brillouin zone are found. This behaviour is the same than that observed experimentally by Inelastic X-Ray scattering. The macroscopic sound speed is obtained for wavenumbers tending to zero. The values obtained with EAM and Morse potentials are in qualitative agreement with those obtained experimentally in systems of similar composition. The Ioffer-Regel limit (IR), where the coherence length of the phonon is similar to the phonon wavelenght, was computed. It is found that the Ioffe-Regel frequency decreases slightly when applying faster quenching rates. The longitudinal Ioffe-Regel limit was found at frequencies higher than the Boson peak frequency for all the cases, although the diference in EAM systems is much reduced. Contrary to the typical results obtained in the LJ systems, using EAM potentials in both Cu20Pd80 and Cu50Pd50 the longitudinal IR limit is very close to the position of the BP while the transversal IR limit is found well below. This behaviour is coherent with that found by IXS measurements. It is inferred that the EAM potential increases the interaction between the longitudinal modes and the BP excess states. Finally the fragility of the studied systems was obtained by calculating the viscosity at different temperatures. Lennard-Jones systems showed a much larger fragility than EAM and Morse systems. However, even systems simulated with more realistic potentials showed fragility values much higher than those obtained experimentally. This is attributed to the extremely high quenching rates used in simulations, as it is known experimentally that fragility increases with the quenching rate. El desenvolupament de nous materials té un gran impacte en totes les àrees de l'enginyeria, i en particular a l'enginyeria aeronàutica. Els vidres metàl·lics son materials relativament nous, amb excel·lents propietats mecàniques; el seu estudi és imprescindible per la seva implantació tecnològica. Les propietats mecàniques macroscòpics d'un material estan determinades per la seva estructura atòmica. En particular, la fractura de materials fràgils s'inicia per la generació de modes de vibració. Als vidres metàl·lics, d'estructura amorfa, l'espectre vibracional té característiques específiques. En aquest treball s'estudien les propietats vibracionals dels vidres metàl·lics emprant simulacions de dinàmica Molecular. L'estudi es centra en sistemes binaris simulats emprant potencials atòmics de Lennard-Jones (LJ), Morse i Embedded Atom method (EAM), de tipus semiempíric. Com als vidres metàl.lics basats en Pd, el quocient de les masses d'ambdues espècies en les simulacions és alt, essent 2 en potencials LJ i 1.67 en potencials Morse i EAM. Simulacions a gran escala permeten simular sistemes amb heterogeneities a escala nm, i la simulació a velocitats de refredament diferents permet obtenir configuracions en diferents estats de relaxació. La dinàmica de les vibracions atòmiques dels vidres metàl·lics és un tema de debat. L'origen de l'excés dels modes de vibració conegut com Boson Peak (BP) no és clar. Als sistemes analitzats s'observa que la dependència de la posició i la intensitat del BP amb la mida del sistema és feble. Al contrari, la intensitat de la BP augmenta amb la velocitat de refredament, mentre que la seva posició es desplaça a freqüències menors. Els resultats obtinguts emprant potencials de realista tipus EAM coincideixen amb les dades experimentals disponibles en vidres de composicions similars. El factor d'estructura dinàmica, S (q, ¿) es calculà també en sistemes grans per obtenir informació sobre el comportament de les excitacions acústiques en vectors d'ona baixos. També es va obtenir la relació de dispersió de fonons longitudinals i transversals. En tots els casos estudiats, l'amplada del pic, GL,T(q), augmenta amb la freqüència. S'observa una regió lineal a baixos nombres d'ona i una flexió quan s'aproxima el límit de la pseudo-zona de Brillouin. Aquest comportament és el mateix que l'observat experimentalment emprant Inelàstic X-Ray scattering (IXS). La velocitat del so macroscòpica s'obté quan el nombre d'ona tendeix a zero. Els valors obtinguts amb potencials EAM i Morse estan d'acord amb els valors mesurats experimentalment en sistemes de composició similar. El límit Ioffe-Regel (IR) es defineix com la freqüència a la que la longitud de coherència del fonó és similar a la longitud d'ona de fonó. S'observa que la freqüència IR augmenta lleugerament als sistemes més relaxats. El límit IR longitudinal és troba en tots els casos a freqüències superiors a la freqüència del BP. Contràriament als resultats obtinguts en els sistemes LJ, el límit longitudinal IR en Cu20Pd80 y Cu50Pd50 és molt a prop de la posició del BP, mentre que el límit transversal IR és molt per sota d'aquest valor; aquests resultats estan d'acord amb els obtinguts experimentalment per IXS. S'infereix que el potencial EAM augmenta la interacció entre modes longitudinals i l'excés de modes del BP. Finalment s'obtingué la fragilitat en els sistemes estudiats mitjançant el càlcul de la viscositat a temperatures diferents. És conegut experimentalment que la fragilitat augmenta amb la velocitat de refredament. Els sistemes LJ mostraren una fragilitat molt més alta que els Morse i EAM. No obstant això, fins i tot els sistemes simulats amb potencials realistes van mostrar valors de la fragilitat molt superiors als obtinguts experimentalment a causa de l'extrema velocitat extrema de refredament de les simulacions El desarrollo de nuevos materiales tiene impacto en todas las áreas de la ingeniería, y en particular en la ingeniería aeronáutica. Los vidrios metálicos son materiales relativamente nuevos, con excelentes propiedades mecánicas; su estudio es imprescindible para su implantación tecnológica. Las propiedades mecánicas macroscópicas de un material están determinadas por su estructura atómica. En particular, la fractura de materiales frágiles se inicia mediante la generación de modos de vibración. En los vidrios metálicos, con estructura amorfa, el espectro vibracional tiene características específicas. En este trabajo es estudian las propiedades vibracionales de los vidrios metálicos usando simulaciones de Dinámica Molecular. Se estudian sistemas binarios utilizando potenciales atómicos de Lennard-Jones (LJ), Morse y Embedded atom method (EAM), de tipo semiempírico. Al igual que en los vidrios metálicos a base de Pd, la relación de las masas de ambas especies en las simulaciones es alta, siendo 2 en potenciales LJ y 1,67 en potenciales Morse y EAM. Las simulaciones a gran escala permiten simular sistemas con heterogeneidades a escala nm, y la simulación a diferente velocidad de enfriamiento permite obtener configuraciones en diferentes estados de relajación. La dinámica de las vibraciones atómicas de los vidrios metálicos es un tema de debate. El origen del exceso de los modos de vibración conocidos como Boson Peak (BP) no es claro. En los sistemas analizados se observa que la dependencia de la posición y la intensitat del BP con el tamaño del sistema es débil. Por el contrario, la intensidad del BP aumenta con la velocidad de enfriamiento, mientras que su posición se desplaza a frecuencias menores. Los resultados obtenidos mediante el uso de potenciales realistas tipo EAM coinciden con los datos experimentales disponibles en vidres de composiciones similares. El factor de estructura dinámica, S (q, ω) se calculó también en sistemas grandes para obtener información sobre el comportamiento de las excitaciones acústicas en vectores de onda bajos. También se obtuvo la relación de dispersión de fonones longitudinal read less

Abstract: The self-diffusion in very thin Cu (001) film that formed by 2~11 atomic layers have been studied by using modified analytic embedded atom method (MAEAM) and a molecular dynamic (MD) simulation. The vacancy formation is the most easily in of Cu (001) thin film formed by any layers. The vacancy formation energy 0.5054eV in of the Cu (001) thin film formed by layers is the highest in all the values in the ones that formed by layers. The vacancy in and 3 is easily migrated to layer, and the vacancy in is easily migrated in intra-layer, and the vacancy in is easily migrated to when the corresponding atomic layer is existed. The vacancy formation and diffusion will not be affected by the atomic layer when the Cu (001) thin film is formed by more than ten layers (). read less

Abstract: We present a method for determining the melting point of copper nanowires based on classical molecular dynamics simulations and use it to investigate the dependence of the melting point on wire diameter. The melting point is determined as the temperature at which there is a significant change in the fraction of liquid atoms in the wire, according to atomic bond angle analysis. The results for the wires with diameters in the range 1.5 nm to 20 nm show that the melting point is inversely proportional to the diameter while the cross-sectional shape of the wire does not have a significant impact. Comparison of results obtained using different potentials show that while the absolute values of the melting points may differ substantially, the melting point depression is similar for all potentials. The obtained results are consistent with predictions based on the semi-empirical liquid drop model. read less

Abstract: Carrying out molecular dynamics (MD) simulations is a relevant way to understand growth phenomena at the atomic scale. Initial conditions are defined for reproducing deposition conditions of plasma sputtering experiments. Two case studies are developed to highlight the implementation of MD simulations in the context of plasma sputtering deposition: ZrxCu1−x metallic glass and AlCoCrCuFeNi high entropy alloy thin films deposited onto silicon. Effects of depositing atom kinetic energies and atomic composition are studied in order to predict the evolution of morphologies and atomic structure of MD grown thin films. Experimental and simulated x-ray diffraction patterns are compared. read less

Abstract: The synthesis of metal nanoparticles by ultrafast laser ablation of nanometers-thick metal films has been studied experimentally and computationally. Near-threshold backside laser ablation of 2–20 nm-thick Pt films deposited on fused silica substrates was found to produce nanoparticles with size distributions that were bimodal for the thicker films, but collapsed into a single mode distribution for the thinnest film. Time-resolved imaging of blackbody emission from the Pt nanoparticles was used to reveal the nanoparticle propagation dynamics and estimate their temperatures. The observed nanoparticle plume was compact and highly forward-directed with a well-defined collective velocity that permitted multiple rebounds with substrates to be revealed. Large-scale molecular dynamics simulations were used to understand the evolution of compressive and tensile stresses in the thicker melted liquid films that lead to their breakup and ejection of two groups of nanoparticles with different velocity and size distributions. Ultrafast laser irradiation of ultrathin (few nm) metal films avoids the splitting of the film and appears to be a method well-suited to cleanly synthesize and deposit nanoparticles from semitransparent thin film targets in highly directed beams. read less

Abstract: The growth of single-walled bimetallic Au-Ag, Au-Cu and Ag-Cu alloy nanotubes (NTs) and nanowires (NWs) in confined carbon nanotubes (CNTs) has been investigated by using the classical molecular dynamics (MD) method. It is found that three kinds of single-walled gold-silver, gold-copper and silver-copper alloy NTs could indeed be formed in confined CNTs at any alloy concentration, whose geometric structures are less sensitive to the alloy concentration. And an extra nearly pure Au (Cu) chain will exist at the center of Au-Ag (Au-Cu and Ag-Cu) NTs when the diameters of the outside CNTs are big enough, thus producing a new type of tube-like alloy NWs. The bonding energy differences between the mono- and hetero-elements of the coinage metal atoms and the quasi-one-dimensional confinement from the CNT play important roles in suppressing effectively the "self-purification" effects, leading to formation of these coinage alloy NTs. In addition, the fluid-solid phase transition temperatures of the bimetallic alloy NTs are found to locate between those of the corresponding pure metal tubes. Finally, the dependences of the radial breathing mode (RBM) frequencies and the tube diameters of the alloy NTs on the alloying concentration were obtained, which will be very helpful for identifying both the alloying concentration and the alloy tube diameters in future experiments. read less

Abstract: Shock responses of graphene reinforced composites are investigated using molecular dynamics simulations. The first case studied is the response of spaced multilayer graphene plates under normal impact of a spherical projectile, focusing on the effect of the number of graphene monolayers per plate on the penetration resistance of the armor. The simulation results indicate that the penetration resistance increases with decreasing number of graphene monolayers per plate. The second case studied is the penetration resistance of laminated copper/graphene composites. The simulation results demonstrate that under normal impact by a spherical projectile the penetration resistance of copper can be improved significantly by laminating the copper plates with graphene. The results of this research have revealed the possibility that graphene might be used in hyper velocity-relevant armor systems to enhance their penetration resistance. read less

Abstract: The aim of this work is to investigate structure and stress evolution in Au/Cu bilayer systems during deposition. The approach used here is based on an embedded atom method (EAM). interatomic potential database for different metal elements, their alloys and multilayers. We applied the kinematical scattering theory to calculate the X-ray scattering profiles. In this case the X-ray scattering techniques are used for the structural characterization of crystal structures obtained from simulation data. This method was applied to determine the lattice parameters in any directions. The lattice parameters in deposited layers were directly determined by the analysis of X-ray diffraction profiles. Results shows that on the interface of Au/Cu system, the crystalline lattice of Au layer is fitted to crystalline lattice of Cu layer. We found that deformation of the crystal lattice near the interface has a major influence on the stress. read less

Abstract: A molecular dynamics simulation based on the embedded-atom method was conducted at different sizes of single-crystal Ag nanoparticles (NPs) with diameters of 4 to 20 nm to find complete melting and surface premelting points. Unlike the previous theoretical models, our model can predict both complete melting and surface premelting points for a wider size range of NPs. Programmed heating at an equal rate was applied to all sizes of NPs. Melting kinetics showed three different trends that are, respectively, associated with NPs in the size ranges of 4 to 7 nm, 8 to 10 nm, and 12 to 20 nm. NPs in the first range melted at a single temperature without passing through a surface premelting stage. Melting of the second range started by forming a quasi-liquid layer that expanded to the core, followed by the formation of a liquid layer of 1.8 nm thickness that also subsequently expanded to the core with increasing temperature and completed the melting process. For particles in the third range, the 1.8 nm liquid laye... read less

Abstract: The structure and properties simulation of Cu under vacuum were studied by ab initio molecular dynamics simulation. The calculation results were characterized in terms of radial distribution function (RDF), coordination number (CN) and partial density of states (PDOS). The results show that the average distance between atoms increased with the temperature, while CN decreased, which indicated an obvious improvement of the thermal motion between atoms. The simulation datas showed that the liquid phase appeared in the system when the temperature arrived 1373K,which close to the melting point(1357K) of copper. read less

Abstract: High velocity impact of copper plates has been studied using the molecular dynamics code LAMMPS. The impact of copper plates at 1100 m/s impact velocity shows that spall of the material takes place at 160 kbar tensile pressure. Void nucleation is not observed at lower impact velocities where the peak tensile pressure stays below 160 kbar. No significant void nucleation occurs in the neighbouring regions where the peak tensile pressure stays below 160 kbar. For impact velocities close to the threshold, we observe stochastic behaviour of the spall process with respect to small changes in the initial atomic coordinates. read less

Abstract: Today, there is a need to understand the micro mechanism of material removal to achieve a better roughness in ultra precision machining (UPM). The conventional finite element method becomes impossible to use because the target region and grids are very tiny. In addition, FEM cannot consider the micro property of the material such as atomic defect and dislocation. As an alternative, molecular dynamics (MD) simulation is significantly implemented in the field of nano-machining and nano-tribological problems to investigate deformation mechanism like work hardening, stick-slip phenomenon, frictional resistance and surface roughness [1]. One of the machining parameters than can affect nano-cutting deformation and the machined surface quality is tool nose radius [2]. In this paper molecular dynamics simulations of the nano-metric cutting on single-crystal copper were performed with the embedded atom method (EAM). To investigate the effect of tool nose radius, a comparison was done between a sharp tool with no edge radius and tools with a variety of edge radii. Tool forces, coefficient of friction, specific energy and nature of material removal with distribution of dislocations were simulated. Results show that in the nano-machining process, the tool nose radius cannot be ignored compared with the depth of cut and the edge of tool can change micro mechanism of chip formation. It appears that a large edge radius (relative to the depth of cut) of the tool used in nano-metric cutting, provides a high hydrostatic pressure. Thus, the trust force and frictional force of the tool is raised. In addition, increasing the tool edge radius and the density of generated dislocation in work-piece is scaled up that is comparable with TEM photographs [6]. read less

Abstract: Nowadays, the nano-machining process is used to produce high quality finished surfaces with precise form accuracy. To understand and analyze the chip formation mechanism of nano-machining process on an atomistic scale, since the experimentation is not an easy task, numerical simulation such as molecular dynamic (MD) simulation is a very useful method. In this paper, MD simulation of the nano-metric cutting of single-crystal copper was performed with a single crystal diamond tool. The model was solved with both pair wise Morse potential function and embedded atom method (EAM) potential to simulate the inter-atomic force between the work-piece and a rigid tool. The chip formation mechanism, dislocation generation, tool forces and generated temperature were investigated. Results show that the Morse potential cannot perform an appropriate defect formation and plastic deformation in nano-metric cutting of metals. Also, tool forces in Morse potential are more than the forces in EAM potential. Furthermore, the fluctuations of resultant forces in Morse potential are greater than that of EAM. In addition, using many-body interaction potentials like EAM can lead to substantial changes in surface energies, elastic-plastic properties and atomic displacement, compared with the pair-wise potentials like Morse. Finally, the atomic displacement investigation shows that in EAM potential study, only the atoms in a local region near the cutting process are displaced, but in Morse potential a large portion of atoms has affected during cutting process. Subsequently, the chip temperature in EAM potential is more than that of Morse potential. read less

Abstract: The uniaxial tensions of single crystal Cu nanowires (NWs) with different circular cross-section radiuses are simulated by molecular dynamics (MD) method. Atomic interactions in Cu NWs are described by EAM potential. The results show that the plastic deformation of NWs under uniaxial tension is dominantly controlled by dislocation nucleation and gliding. The mechanical properties of NWs are size-dependent. The NWs with larger radius of cross-section possess higher strength and ductility. Furthermore, the site of neck formation and following break of NWs has strong dependence on size scale of NWs. read less

Abstract: A hybrid method combing molecular dynamics and two-step radiation heating model is used to study the kinetics and microscopic mechanisms of picosecond laser melting of monocrystalline copper in stress confinement regime. The nonequilibrium processes of laser melting are simulated by classical MD method, and laser excitation as well as subsequent relaxation of the conduction band electrons are described continually by two-step radiation heating model. The mechanism responsible for melting of copper under picosecond laser pulse irradiation can be attributed to homogeneous nucleation of the liquid phase inside the solid region. The speed of stress wave is predicted to be 4400m/s equal to that of sound. The liquid and crystal regions are identified definitely in the atomic configurations by means of Local Order Parameter, in-plane structure and number density of atoms. Velocity-reducing technique is proved efficient in avoiding the influence of the reflected stress wave on melting process by comparing two models with velocity-reducing technique and free boundary condition at the bottom respectively. read less

Abstract: We present the results of density-functional-theory-based calculations for the activation energies for the diffusion of adatoms (Cu or Ag) on Cu(100) and Ag(100) with and without steps. We find that only for Cu on Ag(100), exchange is the dominant mechanism for the diffusion on terraces. On the other hand, for diffusion at step edges, exchange is the dominant mechanism except for Ag on Cu(100). This result also indicates that incorporation of Cu atoms into the step edges of Ag(100) costs only 330 meV, while the energy cost for Ag incorporation into Cu(100) step edge is much higher (about 700 meV). We find the hierarchy of Ehrlich-Schwoebel barriers to be: 170 meV for Ag on Cu(100); 60 meV for Cu on Cu(100); 20 meV for Ag on Ag(100), and $\ensuremath{-}30\text{ }\text{meV}$ $(\ensuremath{-}270\text{ }\text{meV})$ for Cu on Ag(100). These barriers point to a striking difference in the growth modes for Ag layers on Cu(100) and Cu layers on Ag(100). read less

Abstract: The surface relaxation and the rumpling of the top and the 2nd layer together with the mean thermal vibration amplitudes of NiAl(110) are determined by high-resolution medium energy ion scattering (MEIS) with an excellent depth resolution of typically ±0.01 A. We also perform classical molecular dynamics (MD) simulations employing the embedded atom method and the first principles calculations using the VASP (Vienna Ab-initio Simulation Package) code. The results obtained by MEIS are compared with the theoretical predictions and experimental analysis reported so far. Interestingly, the present MEIS analysis observes slightly expanded relaxation 12 e Δ , which is supported by the present MD and VASP calculations and by X-ray diffraction analysis, whereas other experimental and theoretical analyses give contracted relaxation. The root mean square thermal vibration amplitude of the bulk Ni atoms is determined to be 0.10±0.005 A, which agrees well with the value of 0.097 A derived from the phonon dispersion relation calculated from VASP. A slightly enhanced thermal vibration amplitude of the top layer Ni in the surface normal direction observed is consistent with the MD simulation. 1 Department of Physics, Ritsumeikan University, Kusatsu, Shiga-ken 525-8577, Japan 2 Advanced Industrial Science and Technology, AIST, Ikeda, Osaka 563-8577, Japan read less

Abstract: The nanoscale contacts, which play a key role in nanotechnology and micro-/ nanoelectromechanical systems, are fundamentally important for a wide range of problems including adhesion, contact formation, friction and wear, etc. Because continuum contact mechanics has limitations when it is applied at length of nanoscale, molecular dynamics (MD) simulations, which can investigate internal physical mechanisms of nanostructures by atomic motions in detail, become one of the most promising approaches for investigating mechanical behaviors of contacts in nanoscale. First, contacts between rigid cylindrical probes with different radii and an elastic half-space substrate are studied by using MD simulations with the assistance of the classical Lennard-Jones potential. For contacts without adhesion, the relationship between the applied force and the contact half-width is analyzed. The von Mises stress distributions are then discussed. For contacts with adhesion, the phenomena of the jump-to-contact, the break-off contact, and the hysteresis are observed. The pressure distributions and the von Mises stress contours in the contact region agree with the existing solutions. Second, the effects of the surface topography on adhesive contacts are studied by using MD simulations with the embedded atom method potential. The adhesive contact mechanical characteristic of a series of asperities with different shapes, different sizes, and different numbers on contacting surfaces are discovered and compared. The results show that the surface topography is one of the major factors, which may influence the contact behaviors between the interfaces of nanoscale components. read less

Abstract: Ab initio molecular dynamics based on density functional theory within the generalized gradient approximation was used to explore decomposition on Al(111) of butanol-alcohol and butanoic-acid, two important boundary additives in Al processing. Each molecule was oriented with its functional group closest to the surface and then given an initial velocity toward the surface. Decomposition occurred upon collision with Al(111) resulting in the formation of adhered fragments that represent the very initial stages in additive film formation during plastic deformation where nascent Al is liberated. Bonding interactions over the simulation time frames were explored with contours of the electron localization function. Results of the simulations were compared with existing experimental studies of chemical decomposition on clean Al surfaces and found to be in qualitative accord. The effects of other initial molecular orientations on decomposition were explored in ancillary calculations where the molecules were rotated through 90 deg. and 180 deg. prior to collision with Al(111) read less

Abstract: The structure and binding energy of copper clusters of the size range 70 to 150 were studied by using the embedded-atom method. The stability of the structure of the clusters was studied by calculating the average binding energy per atom, first difference energy and second difference energy of copper cluster. Most of the copper clusters of the size n = 70–150 adopt an icosahedral structure. The results show that the trends are in agreement with theoretic prediction for copper clusters. The most stable structures for copper clusters are found at n = 77, 90, 95, 131, 139. read less

Abstract: Defects in free surfaces are expected to be seeds for the nucleation of dislocations, which is the likely way nanoscale materials suffer plastic deformation. The nucleation results in the competition between the image force attracting the dislocation to the surface and the applied strain. In this work, two methods based on molecular dynamics simulations using an embedded atom method (EAM) potential are used to determine the activation energy and the critical radius for the formation of dislocations from a surface defect in a typical fcc metal. read less

Abstract: Nanoscale rods have been shown to exhibit a multiple twinned structure. The rods grow along a [110]-type crystallographic direction and have a pentagonal cross section with five (111) twins connecting the wire center to the corners of the pentagon. Here, we use molecular dynamics simulations with an embedded atom method interatomic potential for Ag to compute the ground-state energies of the multitwinned rods and compare with the bulk equilibrium crystal shape, as estimated from a Wulff construction. The excess energy of the nontwinned equilibrium nanorods and the multitwinned nanorods was obtained as a function of the wire length $(L)$ as well as the cross sectional area $({A}_{\text{cs}})$. Various contributions to the total energy, such as twin boundary energy and surface energies, are discussed and included in an analytical model that compares favorably with the simulation results. Our results show that for infinitely long nanowires with ${A}_{\text{cs}}l1500\text{ }{\text{nm}}^{2}$, the nontwinned structure is always energetically favorable. However, if the energy of the dipyramidal atomic structure at the nanorod ends is included in the model then the twinned nanorods are stable with respect to the nontwinned rods below a critical aspect ratio $(L/\sqrt{{A}_{\text{cs}}})$. read less

Abstract: Atomic behavior during ion beam sputtering was investigated by using classical molecular dynamics simulation. When Ar ion bombards on Au and Pd(001) surface with various incidence energies and angles, some atoms which gained substantial energy by impacting Ar ion were sputtered out and, simultaneously, others were landed on the surface as if surface atoms were redeposited. It was observed that the redeposited atoms are five times for Au and three times for Pd as many as sputtered atoms irrespective of both incidence energy and angle. From sequential ion bombarding calculations, contrary to the conventional concepts which have described the mechanism of surface pattern formation based only on the erosion theory, the redeposition atoms were turned out to play a significant role in forming the surface patterns. read less

Abstract: The formation of two conjoint fivefold deformation twins (DTs) in copper nanowires under bending is reported based on molecular dynamics simulations. It is found that an intermediate icosahedral phase is formed to facilitate the transformation from a low dense (010) plane in a face-centered-cubic lattice to a {111} close-packed fashion, forming trijunctions composed of three DTs. These trijunctions can easily interact with other DTs, forming two conjoint fivefold DTs. This formation process differs from the one observed by Cao and Wei [Appl. Phys. Lett. 89, 041919 (2006)], that is, fivefold DTs could be formed without introducing initial imperfections in simulations. read less

Abstract: Molecular dynamics simulation is used to investigate the mechanical properties of infinitely long, cylindrical bimetallic Pd-Pt nanowires, with an approximate diameter of $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and two different compositions (25% and 50% Pt). The nanowires are subjected to uniaxial tensile strain along the [001] axis with varying strain rates of $0.05%\phantom{\rule{0.2em}{0ex}}{\mathrm{ps}}^{\ensuremath{-}1}$, and $5.0%\phantom{\rule{0.2em}{0ex}}{\mathrm{ps}}^{\ensuremath{-}1}$, at simulation temperatures of 50 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, to study the effects of strain rates and thermal conditions on the deformation characteristics and mechanical properties of the nanowire. The deformation and rupture mechanism of these nanowires is explored in detail. Comparisons to the behavior exhibited by pure Pd and Pt nanowires of similar diameter are also made. The effect of lattice mismatch on the observed deformation modes in bimetallic nanowires is also discussed. Our simulations indicate that Pd-Pt alloy nanowires of various compositions, with little lattice mismatch between Pd and Pt atoms, undergo similar deformation and rupture upon uniaxial stretching. It is found that yielding and fracture mechanisms depend on the applied strain rate as well as atomic arrangement and temperature. At low temperature and strain rate, where crystal order and stability are highly preserved, the calculated stress-strain response of pure Pt and Pd as well as Pd-Pt alloy nanowires showed clear periodic, stepwise dislocation-relaxation behavior. Crystalline to amorphous transformation takes place at high strain rates $(5%\phantom{\rule{0.2em}{0ex}}{\mathrm{ps}}^{\ensuremath{-}1})$, with amorphous melting detected at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Deformation of nanowires at higher strain rates and low temperature, where the superplasticity characteristic is significantly enhanced, results in the development of a multishell helical structure. Mechanical properties of the alloy nanowires are significantly different from those of bulk phase and are dictated by the applied strain rate, temperature, alloy composition, as well as the structural rearrangement associated with nanowire elongation. We find that Young's modulus of both the single component as well as alloy nanowires depends on the applied strain rate and is about 70%--75% of the bulk value. Ductility of the studied nanowires showed a nonmonotonic variation with Pd composition at low strain rates and was significantly enhanced for wires developing and rearranging into a multishell helical structure which occurred at higher strain rates. The Poisson ratio of Pd rich alloys is 60%--70% of its bulk value, whereas that of Pt rich alloys is not significantly changed at the nanoscale. The calculated differences in the nanowire mechanical properties are shown to have significant effect on their applicability in areas such as sensing and catalysis. read less

Abstract: Molecular-dynamics and Monte Carlo simulations have been used to compute the crystal-melt interface stress (f) in a model Lennard-Jones (LJ) binary alloy system, as well as for elemental Si and Ni modeled by many-body Stillinger-Weber and embedded-atom-method (EAM) potentials, respectively. For the LJ alloys the interface stress in the (100) orientation was found to be negative and the f vs composition behavior exhibits a slight negative deviation from linearity. For Stillinger-Weber Si, a positive interface stress was found for both (100) and (111) interfaces: f{100}=(380+/-30)mJ/m{2} and f{111}=(300+/-10)mJ/m{2}. The Si (100) and (111) interface stresses are roughly 80 and 65% of the value of the interfacial free energy (gamma) , respectively. In EAM Ni we obtained f{100}=(22+/-74)mJ/m{2}, which is an order of magnitude lower than gamma. A qualitative explanation for the trends in f is discussed. read less

Abstract: The favorable position of an adatom and the formation energies of a single vacancy and an adatom‐vacancy pair in three low‐index surfaces of body‐centered cubic (BCC) transition metals have been calculated by using the modified analytical embedded atom method (MAEAM). The favorable position of an adatom is at the fourfold and twofold positions above the (100) and (110) surfaces respectively, but it is deviated $(3 - \sqrt{6})a/3$ from the threefold position of the (111) surface. Either the heights of the adatom from the top atomic layer, or the formation energies of a single vacancy, or an adatom‐vacancy pair decrease in sequence of the (110), (100) and (111) surfaces for each metal. Furthermore, the formation energy of an adatom‐vacancy pair is always lower than that of a single vacancy for each low‐index surface of each metal, which shown the formation of adatom‐vacancy pair is more energetically favorable than the vacancy for the BCC transition metals. Copyright © 2007 John Wiley & Sons, Ltd. read less

Abstract: Ferromagnetic L10 ordered alloys are extensively studied nowadays as good candidates for high density magnetic storage media due to their high magnetic anisotropy, related to their chemical order anisotropy. Epitaxial thin bilayers NiPt/FePt/MgO(001) have been grown at 700 K and annealed at 800 K and 900 K. At 800 K, the L10 long-range order increases without measurable interdiffusion. At 900 K, the interdiffusion takes place without destroying the L10 long-range order. This surprising observation can be explained by different diffusion mechanisms that are energetically compared using molecular dynamics simulations in CoPt in the second moment tight binding approximation. In addition, the frequencies of the normal modes of vibration have been measured in FePd, CoPt and FePt single crystals using inelastic neutron scattering. The measurements were performed in the L10 ordered structure at 300 K. From a Born-von Karman fit, we have calculated the phonon densities of states. The migration energies in the 3 systems have been estimated using the model developed by Schober et al. (1981). The phonon densities of states have also been used to calculate several thermodynamic quantities as the vibration entropy and the Debye temperature. read less

Abstract: Thermal characteristics of $\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Pt}$ metal nanowires with diameters ranging from 2.3 to $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and of several compositions were studied by molecular dynamics simulations utilizing the quantum Sutton-Chen potential function. Monte Carlo simulations employing bond order simulation model were used to generate the initial wire configurations that consisted of surface segregated structures. Melting temperatures were estimated based on variations in thermodynamic properties such as potential energy and specific heat capacity. We find that the melting transition temperatures for the nanowires are much lower than those of bulk alloys of the same composition and at least $100--200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ higher than those of nanoclusters of the same diameter. Density distributions along the nanowire cross section and axis as well as components of shell-based diffusion coefficients and velocity autocorrelation functions were used to investigate the melting mechanism in these nanowires. Our findings indicate a surface-initiated melting process characterized by predominantly larger cross-sectional movement. This two-dimensional surface melting mechanism in nanowires differs from that in nanoclusters in which atomic movement is more isotropic in all three dimensions. Differences in the surface melting mechanism result in structural transformations from fcc-hcp type and lead to simulated phase boundaries for nanowires that are different from bulk alloys as well as from same-diameter nanoclusters. A composition and temperature dependent fcc-hcp transformation occurs prior to the melting transition in both nanowires and nanoclusters. Hcp phase occurs over a wider temperature range at Pd-rich compositions and a narrower range at low Pd compositions with the fcc-hcp and hcp-liquid transition temperatures showing a minimum at 25% Pt composition. In contrast, the nanoclusters exhibit a near-linear dependence of melting temperature on Pd composition with the hcp phase existing over a much narrower range of temperatures, closer to the melting transition. Thermal stability of the solid phases of these nanowires was investigated by simulating two alternative starting configurations such as a hypothetical hcp and an annealed-solid structure for two compositions. The size and composition dependence of nanowire melting temperatures are consistent with those predicted by available melting theories. read less

Abstract: Molecular dynamics simulations, based on embedded-atom method potentials, have been used to compute thermodynamic and kinetic properties of crystal–melt interfaces in the bcc metals Mo and V. The interfacial free energy and its associated crystalline anisotropy have been obtained with the capillary fluctuation method and for both metals the anisotropy and the value of the Turnbull coefficient are found to be significantly lower than for the case of fcc materials. The interface mobility, or kinetic coefficient, which relates the isothermal crystallization rate to interface undercooling, was computed by non-equilibrium molecular dynamics simulations. Mobilities in the range 9-16 cm s−1K−1 are obtained. For Mo the mobility in the (110) crystallographic growth direction is larger than in the (100) and (111) directions, whereas for V the growth is found to be isotropic within numerical uncertainty. The kinetic-coefficient results are discussed within the framework of a density-functional-based theory of crystal growth. read less

Abstract: Fivefold deformation twins were reported recently to be observed in the experiment of the nanocrystalline face-centered-cubic metals and alloys. However, they were not predicted previously based on the molecular dynamics (MD) simulations and the reason was thought to be a uniaxial tension considered in the simulations. In the present investigation, through introducing pretwins in grain regions, using the MD simulations, the authors predict out the fivefold deformation twins in the grain regions of the nanocrystal grain cell, which undergoes a uniaxial tension. It is shown in their simulation results that series of Shockley partial dislocations emitted from grain boundaries provide sequential twining mechanism, which results in fivefold deformation twins. read less

Abstract: Using atomistic simulations of dislocation motion in Ni and $\mathrm{Ni}\text{\ensuremath{-}}\mathrm{Au}$ alloys we report a detailed study of the mobility function as a function of stress, temperature, and alloy composition. We analyze the results in terms of analytic models of phonon radiation and their selection rules for phonon excitation. We find a remarkable agreement between the location of the cusps in the $\ensuremath{\sigma}$-$v$ relation and the velocity of waves propagating in the direction of dislocation motion. We identify and characterize three regimes of dissipation whose boundaries are essentially determined by the direction of motion of the dislocation, rather than by its screw or edge character. read less

Abstract: A systematic comparison between the decomposed virial formula and the Tsai formula shows that they are mathematically equivalent in calculating the overall average stress of an atomistic system. But in the case of calculating local stress distribution, the former gives ambiguous results, e.g. it gives nonzero normal stress at free surfaces and it typically ‘underestimates’ the inhomogeneity of microstructures and deformations in material. With a highly degenerate atomic chain model, we show mathematically that the results obtained by the decomposed virial formula are accurate only if the deformation is homogeneous within the neighbourhood of an interaction-cutoff radius, centred at the atomic site considered. Thus it is worth noting that the Tsai formula is more adequate for calculating both the overall average stress and local stress distribution. read less

Abstract: Recent molecular dynamics simulations of nanocrystals have illustrated the importance of stacking fault width for mechanical behavior and microstructure. Stacking fault SF width is a balance between elastic strain energy and the SF energy. While the latter is a material constant, the former strain energy may vary due to local internal stresses thereby effecting SF width. The presence and intensity of these stresses are functions of dislocation interactions in the crystal. Recent high resolution electron microscopy observations reveal a range of narrow SF widths within grains of different sizes including those less than 10 nm. To understand the reason for the presence of dislocations with small SF widths in metals, we first demonstrate theoretically and numerically with molecular statics simulation, the reductions in SF width due to dislocation stress screening. Different dislocation arrangements are examined including a dislocation wall. Second, the reduction in SF width in a thin film is examined via the introduction of free surfaces. These results reveal a wide variation of SF widths depending on structure and indicate that stress screening is an important mechanism for creating the narrow widths observed experimentally. read less

Abstract: The behaviors of a material are nonlinear in the large deformed region. The hyper elastic models can describe such non linear materials. If the hyper elastic material is applied to the hydrostatic tensile load, the void begins to grow when the load exceed the critical value. It is important to study the coalescence of the void growth in order to consider the destruction of the material. In this paper, the void growth simulations in the hyper-elastic material with multiple seeds are studied. The unit rectangular cell with small voids is subjected to the hydrostatic tensile load. This problem can be analyzed by FEM. However, the simulation with the larger number of the voids is not possible. Thus, the CA (Cellular Automaton) is used to describe the behaviors of the void coalescence and the possibility of CA is discussed. read less

Abstract: The initial oxidation stages of Cu(100), (110), and (111) surfaces have been investigated by using in situ ultra-high-vacuum transmission electron microscopy (TEM) techniques to visualize the nucleation and growth of oxide islands. The kinetic data on the nucleation and growth of oxide islands shows a highly enhanced initial oxidation rate on the Cu(110) surface as compared with Cu(100), and it is found that the dominant mechanism for the nucleation and growth is oxygen surface diffusion in the oxidation of Cu(100) and (110). The oxidation of Cu(111) shows a dramatically different behavior from that of the other two orientations, and the in situ TEM observation reveals that the initial stages of Cu(111) oxidation are dominated by the nucleation of oxide islands at temperatures lower than 550 °C, and are dominated by two-dimensional oxide growth at temperatures higher than 550 °C. This dependence of the oxidation behavior on the crystal orientation and temperature is attributed to the structures of the oxygen-chemisorbed layer, oxygen surface diffusion, surface energy, and the interfacial strain energy. read less

Abstract: Bimetallic nanoclusters are of interest because of their utility in catalysis and sensors. The thermal characteristics of bimetallic Pt-Pd nanoclusters of different sizes and compositions were investigated through molecular dynamics simulations using quantum Sutton-Chen (QSC) many-body potentials. Monte Carlo simulations employing the bond order simulation model were used to generate minimum energy configurations, which were utilized as the starting point for molecular dynamics simulations. The calculated initial configurations of the Pt-Pd system consisted of surface segregated Pd atoms and a Pt-rich core. Melting characteristics were studied by following the changes in potential energy and heat capacity as functions of temperature. Structural changes accompanying the thermal evolution were studied by the bond order parameter method. The Pt-Pd clusters exhibited a two-stage melting: surface melting of the external Pd atoms followed by homogeneous melting of the Pt core. These transitions were found to depend on the composition and size of the nanocluster. Melting temperatures of the nanoclusters were found to be much lower than those of bulk Pt and Pd. Bulk melting temperatures of Pd and Pt simulated using periodic boundary conditions compare well with experimental values, thus providing justification for the use of QSC potentials in these simulations. Deformation parameters were calculated to characterize the structural evolution resulting from diffusion of Pd and Pt atoms. The results indicate that in Pd-Pt clusters, Pd atoms prefer to remain at the surface even after melting. In addition, Pt also tends to diffuse to the surface after melting due to reduction of its surface energy with temperature. This mixing pattern is different from those reported in some of the earlier studies on melting of bimetallics. read less

Abstract: We have studied the adsorption characteristics of atomic and molecular oxygen, incident on the Cu(100) surface. Our pseudopotential first-principles calculations yield trajectories for the ${\mathrm{O}}_{2}$ molecule without dissociation barriers at the entrance channel. We discuss the energetics of the ${\mathrm{O}}_{2}$ adsorption and dissociation in terms of the elbow plots which are two-dimensional cuts of the full six-dimensional potential-energy surface. The top site is found to be the most reactive one while at the fcc site molecular adsorption takes place. The adsorption energies at horizontal configurations of the ${\mathrm{O}}_{2}$ molecule are found to be larger than those at the vertical configurations. The local densities of states reveal differences between the sites with direct dissociative adsorption and the ones with molecular precursor states. We also discuss the interactions between O and Cu atoms adsorbed at the hollow sites on Cu(100), and the corresponding diffusion barriers. read less

Abstract: Anisotropy in grain boundary “thermo-kinetics” is central to our understanding of microstructural evolution during grain growth and recrystallization. This paper focusses on role of atomic-scale computer simulation techniques, in particular molecular dynamics (MD), in extracting fundamental grain boundary properties and elucidating the atomic-scale mechanisms that determine these properties. A brief overview of recent strides made in extraction of grain boundary mobility and energy is presented, with emphasis on plastic strain induced boundary motion (p-SIBM) during recrystallization and curvature driven boundary motion (CDBM) during grain growth. Simulations aimed at misorientation dependence of the grain boundary properties during p-SIBM and CDBM show that boundary mobility and energy exhibit extrema at high symmetry misorientations and boundary mobility is comparatively more anisotropic during CDBM. This suggests that boundary mobility is dependent on the driving force. Qualitative observations of the atomic-scale mechanisms in play during boundary motion corroborate the simulation data. p-SIBM is dominated by motion of dislocation-interaction induced stepped structure of the grain boundaries, while correlated shuffling of group of atoms preceded by rearrangement of grain boundary free volume due to single atomic-hops across the grain boundary is frequently observed during CDBM. Comparison of the simulation results with high-purity experimental data extracted in Al indicates that while there is excellent agreement in misorientation dependent anisotropic properties, there are significant differences in values of boundary mobility and migration activation enthalpy. This strongly suggests that minute concentration of impurities retard grain boundary kinetics via impurity drag. Finally, the paper briefly discusses current and future challenges facing the computer simulation community in studying grain boundary systems in real materials where extrinsic effects (vacancy, impurity, segregation and particle effects) significantly alter the microscopic structure-mechanism relations and play a decisive role in determining the boundary properties. read less

Abstract: Density functional theory calculations of adsorption of chlorine at the perfect and defective silver (111) surface have shown that the energies of adsorption of chlorine atoms show little variation (less than 30 kJ mol - 1 ) between the different sites, from -136 kJ mol - 1 next to a silver adatom, through -159 kJ mol - 1 at the perfect surface to - 166 kJ mol - 1 next to a silver vacancy at the surface. Molecular chlorine adsorbs in a series of energetically similar overlayers, which are in good agreement with experimentally found structures. The lowest energy configuration is a planar hexagonal honeycomb structure of chlorine atoms adsorbed in fcc and hep hollow sites on the silver surface. An energetically similar structure is geometrically nonplanar, but has a planar electronic structure. Although the chlorine molecules are virtually dissociated (Cl-Cl distance = 3.40 A), significant electron density is distributed along the Cl-Cl axes, leading to a network of electronic interactions between the adsorbed chlorine atoms. The adsorption energy for Cl 2 is calculated at -231 kJ mol - 1 , in good agreement with experiment. Calculated Ag-Cl bond lengths of 2.69, 2.47, and 2.33 A agree with several experimental studies and show that the different bond lengths found experimentally are not anomalous, but due to the formation of geometrically different but energetically almost identical chlorine overlayer structures. read less

Abstract: Multilayer relaxations, surface/interface energies, and hydrogen binding sites and energies were systematically studied using the embedded-atom method, for a series of the clean and hydrogenated Ni surfaces and symmetrical tilt Ni grain boundaries. The hydrogen binding energies were in the range of 2.7-2.9 eV at the surface sites while 2.1-2.6 eV at the grain-boundary sites, both being larger than at the fcc crystal interior site, 2.1 eV. These data are consistent with the conclusions from ab initio calculations that intergranular embrittlement is seen by hydrogen segregation at the Ni grain boundaries. It was found that multilayer relaxation of Ni plays a key role in trapping hydrogen at the grain boundaries. read less

Abstract: Structural and magnetic properties of thin Mn films on the Fe(001) surface have been investigated by a combination of photoelectron spectroscopy and computer simulation in the temperature range 300 K≤T ≤750 K. Room-temperature as deposited Mn overlayers are found to be ferromagnetic up to 2.5-monolayer (ML) coverage, with a magnetic moment parallel to that of the iron substrate. The Mn atomic moment decreases with increasing coverage, and thicker samples (4-ML and 4.5-ML coverage) are antiferromagnetic. Photoemission measurements performed while the system temperature is rising at constant rate (dT/dt ∼0.5 K/s) detect the first signs of Mn-Fe interdiffusion at T=450 K, and reveal a broad temperature range (610 K≤T≤680 K) in which the interface appears to be stable. Interdiffusion resumes at T≥680 K. Molecular dynamics and Monte Carlo simulations allow us to attribute the stability plateau at 610 K≤T≤680 K to the formation of a single-layer MnFe surface alloy with a 2 ×2 unit cell and a checkerboard distribution of Mn and Fe atoms. X-ray-absorption spectroscopy and analysis of the dichroic signal show that the alloy has a ferromagnetic spin structure, collinear with that of the substrate. The magnetic moments of Mn and Fe atoms in the alloy are estimated to be 0.8μ B and 1.1μ B , respectively. read less

Abstract: One important wear mechanism involves the shear of asperities by other asperities. Molecular dynamics is used to simulate the shearing of aluminum asperities by a “hard” (Lennard-Jones) asperity. These simulations involve the use of a reliable interatomic potential based on the embedded atom method for aluminum that was developed by fitting a large database of density functional calculated forces and experimental data. The simulations are repeated for a wide range of conditions, including velocities, temperatures, asperity shapes, degree of intersection, crystal orientations and adhesive strengths, to determine their effects on the wear process. The design-of-experiment approach is used to analyze the relative importance of each factor and its interactions. Thermal distributions and mechanical deformation in the residual aluminum substrate during asperity shear are analyzed. The final results show that the most significant factor in determining the wear process is the interasperity bonding. The degree of overlap between two asperities is also important. The temperature, the translational velocity, and the crystal orientation play smaller roles. read less

Abstract: Molecular dynamics (MD) simulations of metallic interface separation are presented. A copper grain boundary interface model with 45-degree tilt lattice misorientation is subjected to normal and tangential separations. Nanoscale porosity and dislocation development within the interface region are monitored during the deformation process. In addition, the elastic relaxation response of the interface model is examined after tensile and shear separations. Atomistic calculations of interface separation are motivated by a proposed framework to characterize fracture through continuum interface separation potentials. These potentials are distinguished from previous continuum models in that discrete atomistics are used to determine a set of nanoscale effects, accounting for the influence of atomic structure and imperfections on interface separation or fracture. Atomistic calculations presented in this work may be used to develop functional forms for the advanced interface separation potentials. Identification of the elastic unloading response after normal and tangential separation shows that a distinction must be made regarding elastic relaxation after normal and tangential separation modes. This is contrary to assumptions made in previous damagedependent interface separation potentials. In general, continuum calculations motivated by discrete nanoscale computations facilitate a potentially more complete description of real material separation processes. read less

Abstract: The initial stages of growth of (001)Cu films on (001)Ag substrates have been investigated using the temperature-accelerated dynamics (TAD) simulation method. The acceleration provided by TAD made it possible to simulate the deposition of Cu on (001)Ag at 77 K using a deposition rate of 0.04 ML/s, which matched previously reported experiments. This simulation was achieved without a priori knowledge of the significant atomic processes. The results showed that the increased in-plane lattice parameter of the pseudomorphic Cu reduces the activation energy for the exchange mode of surface diffusion, allowing short-range terrace diffusion and the formation of compact Cu islands on the second film layer at 77 K. Some unexpected complex surface diffusion processes and off-lattice atomic configurations were also observed. read less

Abstract: With an aim to understanding the fundamental mechanisms underlying chemical mechanical planarization (CMP) of copper, we simulate the nanoscale polishing of a copper surface with molecular dynamics utilizing the embedded atom method. Mechanical abrasion produces rough planarized surfaces with a large chip in front of the abrasive particle, and dislocations in the bulk of the crystal. The addition of chemical dissolution leads to very smooth planarized copper surfaces and considerably smaller frictional forces that prevent the formation of bulk dislocations. This is a first step towards understanding the interplay between mechanistic material abrasion and chemical dissolution in chemical mechanical planarization of copper interconnects. read less

Abstract: A recently developed potential function for covalent materials (Phys. Stat. Sol.B212, 9 (1999)) is used to simulate the surface adsorption, and diffusion of Si adtom and ad-dimer on the Si(001) surface. We calculate the formation energies and diffusion activation energies of several possible binding sites. The predicted stable and metastable configurations and diffusion paths of Si ad-atom and Si ad-dimer on Si(001)-(2×1) surface are in agreement with that from the first principle calculations or experiments. read less

Abstract: The homogeneous nucleation process induced by supercooling liquid Pt has been studied by means of molecular dynamics employing the embedded-atom method for the potential energy. The process was simulated by cooling an equilibrated liquid to a low temperature, while structure analysis was performed during the subsequent time evolution of the system under constant temperature and pressure conditions. In order to investigate the effects of degree of supercooling and cooling rate on crystallization, cooling temperature was varied from 900 to 1300 K and three processes at different cooling rates were studied. The results proved that certain degree of supercooling is necessary for homogeneous nucleation of crystals due to the existence of a free energy barrier in forming critical nuclei from supercooled liquid, as described by the classical theory of homogeneous nucleation. The scale of the degree of supercooling has much effect on the incubation period of homogeneous nucleation and nucleation rate. The crystallization towards fcc and hcp phases takes place in all homogeneous nucleation processes from liquid in our simulations. The progression of crystallization is sensitive to cooling rate. A very high cooling rate has been found to prolong incubation period, decrease nucleation rate, thus suppress crystallization. This may be associated with the phenomenon that high cooling rate prohibits the rearrangement of atoms towards forming a crystal lattice at high temperatures during the early stage of cooling. read less

Abstract: A material system of Al sputtered on crystalline Si was dealt with as one of the simple material models for semiconductor material systems. To investigate qualitative properties of sputtered films on an atomic scale, simulations were conducted by a molecular dynamics (MD) method using two film models; i.e. a deposition model based on MD simulations of sputtering process and a crystal model using a crystalline Al film instead of a deposited one. The surface roughness and porosity, which are defined in this work, were found to decrease with an increase in the incident energy of atoms. Relationships between tensile deformation properties and porosities in simulated thin films were also investigated. Although the porosity was found to affect the tensile strength in the direction parallel to the substrate surface, it was revealed that the tensile strength in the direction perpendicular to the substrate surface was hardly influenced by the difference in the porosity. read less

Abstract: Results from two sets of molecular dynamics simulations are reported. In the first set of simulations a nanoscale tip was used to indent single-crystal gold lattices subjected to external strains. These were carried out to explore possible relationships between nanoindentation curves and elastic properties of uniformly strained films. The changes in the slope of the loading curves reflect the stress state of the sample. In the second set of simulations the use of shallow nanoindentation for mapping nonuniform residual surface stress near a dislocation intersecting a surface was tested. Correlation between the maximum force on the tip and the initial local stresses at the point of indentation were observed. Preliminary atomistic simulations indicate that atomic-force microscopy can be used as a nondestructive, nanoscale probe of the surface stress distributions. read less

Abstract: Molecular dynamics simulations of the formation process of Ni/Pd bilayer films were performed by employing the embedded atom method with high (10 eV) and low (0.1 eV) kinetic energies of incident atoms for two different crystal orientations: fcc [111] and fcc [100] of Ni or Pd substrate. The tendencies of the film formation process and the interdiffusion at the interface were similar in both the [111] and [100] cases. However, the strain at the interface of films on the [100] substrate was larger than that on the [111] substrate. At the case of Pd deposited on Ni [100], the Pd film grew with the [111] symmetry and partially observed Ni[100]-c(16/spl times/2) structure. The crystal orientation of Pd layer varied from the value of the strain in Ni [100] under layer. In the crystal orientation dependencies, the value of strain in the magnetic layer at interfaces derived from simulations corresponds well to the experimental value of perpendicular magnetic anisotropy. read less

Abstract: This paper describes the effect of the material used for a tool on atomic scale indentation and cutting mechanisms of metal workpieces, by means of molecular dynamics simulations. The interatomic force between the tool and workpiece is assumed to be a two-body interatomic potential using parameters based on the ab-initio molecular orbital calculation for a(Cr, Ni)-(C, Si)6H9 atom cluster. Molecular dynamics simulated the atomic scale indentation and cutting process of the chromium and nickel workpieces using the diamond, silicon and diamond-like carbon(DLC) tools. The diamond and DLC tools formed the indentation mark. Young's modulus of the chromium and nickel in indentation simulations was larger than that in experiments. This was qualitatively explained by the effect of the surface energy for the workpiece on the elastic modulus. The machinability of the chromium and nickel with the diamond tool was better than that of the silicon tool in atomic scale cutting simulations. The depth of the cut for the workpieces in nano scale cutting experiments with AFM, was similar to that in atomic scale cutting by molecular dynamics simulations. read less

Abstract: We model indentation of a metal surface by combining an atomistic metal with a hard-sphere indenter. This work provides atomistic imaging of dislocation nucleation during displacement controlled indentation on a passivated surface. Dislocations and defects are located and imaged by local deviations from centrosymmetry. For a Au(111) surface, nucleation of partial dislocation loops occurs below the surface inside the indenter contact area. We compare and contrast these observations with empirical criteria for dislocation nucleation and corresponding continuum elasticity solutions. read less

Abstract: The popularity of molecular dynamics (MD) techniques for the investigation of sputtering processes continues to grow, driven in large measure by the rapid increase in the performance-to-price ratio for modern workstations and high-end personal computers. The ready availability of these inexpensive, powerful computing platforms has encouraged researchers to use MD methods to better understand a variety of problems in sputtering. These include studies of: (1) sputtering induced by complex projectiles: (2) the ejection of small clusters during sputtering: (3) the role of inelastic effects during sputtering: (4) sputtering from complex target materials; and (5) chemical effects during sputtering. Increases in computing power also have made it possible to use more realistic many-body potentials in these simulations. This paper reviews some of the recent literature in these areas, and provides an overview of the progress made in the past few years. read less

Abstract: Dynamic properties of free surfaces in their relaxed configuration obtained by the static relaxation method, are studied in fcc Ni and Al, hcp Ti and the L12 alloy Ni3Al. The (001), (110) and (111) surfaces are analysed for the fcc and Li2 structures and the (0001), (1210) and (1010) surfaces for the hcp structure. Interatomic potentials of the embedded-atom method type are used to calculate vibrational eigenfrequencies obtained through the Einstein and ‘cluster’ approaches for the atoms on the first few layers. read less

Abstract: Based on Born's criteria we studied phase stability and theoretical strength of fcc crystals of copper and nickel under [100] uniaxial loading. The calculation was carried out using a simple and completely analytical embedded atom method (EAM) potential proposed by the present authors. For Cu, the calculated value of its theoretical strength (0.33 ? 1011 dyn?cm-2) agrees well with the experimental value (0.30 ? 1011 dyn?cm-2), while the calculated strain (9.76%) is somewhat larger than the experimental one (2.8%). For Ni, its theoretical strength and strain predicted using the EAM potential are found smaller than those predicted using a pair potential. It is worthy to note that unlike previous calculations, in which pair potentials were used and three unstressed fcc, bcc, and fct structures included (for Ni only fcc state is found stable, while for Cu both fcc and bcc states are predicted stable), in present calculations using EAM potential the [100] primary loading path passes through only two zeroes (a stable unstressed fcc structure and an unstable stress-free bcc structure) either for Cu or for Ni. read less

Abstract: The embedded atom method (EAM) was used to simulate the dynamics of deposition of supported metal clusters on fcc transition metals and the dynamic behavior of surfaces. The formation of metal clusters was observed before deposition and the geometries, stabilities and dynamics of a series of metal clusters were discussed as obtained by EAM and compared with first-principle calculations. The calculations indicate that the structure of a cluster in the vapor phase is governed by its zero kelvin stability. We find that if the cluster forms in the vapor phase, it is difficult to get epitaxial growth since the metal cohesive energy is large and the chester can not be broken by thermal motions. The effect of impurities on the deposition is also discussed. The anharmonicity, roughening and reconstruction of surface were also investigated. Three-body interactions were found to he important to understand these phenomena read less

Abstract: Solute-atom segregation is studied by Monte Carlo simulations for three high-angle symmetrical (002) twist boundaries in Au-1 at. % Pt and Pt-1 at. % Au alloys at T = 850 K. It complements our previous study, that focused mainly on low-angle boundaries in the same alloys. Solute enhancement occurs on the Pt-rich side of the phase diagram, while on the Au-rich side net depletion in solute is observed. Following the trend observed for low-angle boundaries, Au as a solute prefers the structural units of the perfect crystal type, while Pt as a solute is depleted at those sites. The solutc concentration at structural units depends on the planar fraction of those units in the boundary. read less

Abstract: Molecular dynamics (MD) simulations of diffusion in a Σ5(310) [001] Al tilt grain boundary were performed using for the first time three different potentials based on the embedded atom method (EAM). The EAM potentials that produce more accurate melting temperatures also yield activation energies in better agreement with experimental data. Compared to pair potentials, the EAM potentials also give more accurate results. read less

Abstract: Molecular dynamics computer simulations were employed to investigate defect production, atomic mixing and chemical disordering in β-NiA1 owing to 10keV cascades. The embedded atom potentials of Voter and Chen (1989) were employed. Point defect energies and threshold displacement energies were also obtained. The defect production efficiency for the 10 keV cascade was 0.27, which is somewhat larger than that found in simulations of metals with a f.c.c. structure. No interstitial clustering and very little vacancy clustering were observed. The threshold displacement energy is lowest for Ni atoms recoiling along 〈100〉, 34 eV, followed by the 〈110〉 ≊54eV, and 〈111〉, ≊130eV. The 〈111〉 crowd-ion is the stable interstitial defect. The mixing parameter was small, 6.8 A5/eV. Efficient chemical disordering was nevertheless observed near the centre of the cascade; there the short range order parameter was reduced to 0.24. Statistical methods were developed to follow the atomic structure during the evolution ... read less

Abstract: Based on the embedded-atom method, molecular dynamics simulations have been performed to study the structural features of Al and Cu in the liquid and solid states during rapid solidification. The calculated pair correlation functions above the melting points of Al and Cu are found to be in good agreement with experiment, especially for Cu. The results show that the EAM can correctly and efficiently predict the glass transition and crystallization during rapid solidification from liquid metals, and can also describe microstructures of liquids, supercooled liquids, glasses and crystalline phases. In addition, structural analysis using bond orientational order and pair analysis techniques have been made in detail, and the effect of cooling rate on microstructures during rapid solidification has been analysed. read less

Abstract: The structures of point defect clusters of both interstitial and vacancy type were examined by computer simulation using molecular dynamics and molecular statics with the DYNAMO code (Daw, Foiles and Baskes [6]). The code implements an isotropic potential of embedded atom method (EAM) developed by Daw and Baskes [5]. Interstitial clusters relax to either the immobile mixture of dumbbell and bcc interstitials or a mobile platelet of parallel interstitials. The latter cluster moves along directions. A tri-vacancy relaxes to an un-collapsed stacking fault tetrahedron (sft) of Damask-Dienes type (3v-sft) containing a central atom that vibrates with a large amplitude. A hexa-vacancy relaxes to a stacking fault tetrahedron the structure of which fluctuates between a sft and void. Larger vacancy clusters are stable as a combination of sft and 3v-sft. In these vacancy clusters, atoms show significant vibration with large amplitude. Voids form only with the inclusion of gas-atoms into va... read less

Abstract: The reconstructions of high-angle twist grain boundaries on the four densest atomic planes in fcc copper, as described by a Lennard-Jones potential, and gold, as described by an embedded-atom-method potential, are investigated using the recently developed method of grand-canonical simulated quenching. It is found that the grain boundaries on the widely spaced (111) and (100) planes do not reconstruct, while those on the less widely spaced (110) and (113) planes do reconstruct. The effect that reconstruction can have on the physical properties of an interfacial system is illustrated by comparing the elastic properties and ideal cleavage energies of reconstructed grain boundaries with those of corresponding unreconstructed grain boundaries. read less

Abstract: We present a new, nondestructive, method for determining chemical potentials in Monte Carlo and molecular dynamics simulations. The method estimates a value for the chemical potential such that one has a balance between fictitious successful creation and destruction trials in which the Monte Carlo method is used to determine success or failure of the creation/destruction attempts; we thus call the method a detailed balance method. The method allows one to obtain estimates of the chemical potential for a given species in any closed ensemble simulation; the closed ensemble is paired with a ‘‘natural’’ open ensemble for the purpose of obtaining creation and destruction probabilities. We present results for the Lennard‐Jones system and also for an embedded atom model of liquid palladium, and compare to previous results in the literature for these two systems. We are able to obtain an accurate estimate of the chemical potential for the Lennard‐Jones system at higher densities than reported in the literature. read less

Abstract: Stacking faults, complex stacking faults and antiphase boundary in a {113} plane of L10 type TiAl were introduced into the perfect crystal by a shear model. The structure and the stacking sequences of these defects were studied. The energies of these defects were calculated by an embedded atom method. The results show that metastable stacking faults and antiphase boundary exist in the (113) plane while no metastable complex stacking faults can be found. The determined energies of the intrinsic stacking faults, the extrinsic stacking faults and the antiphase boundary by the embedded atom method were 1195, 1217 and 976 (mJm−2), respectively. read less

Abstract: In a combined theoretical and experimental study, the energies and structures of σ = 3, [011] tilt boundaries in Cu were investigated. Equilibrium atomistic structures and grain-boundary energies were calculated by static energy minimization using an embedded-atom potential. Cu bicrystals of the same boundary orientations were fabricated by welding of Cu single crystals. Grain-boundary energies were measured by the thermal grooving technique. The atomistic structure of the {211} twin boundary was investigated by high-resolution transmission electron microscopy (HRTEM). The calculated grain-boundary energies γb plotted against the inclination of the boundary plane show a minimum for the {111} twin boundary and a second minimum at an inclination of about 82° to the {111} boundary. The calculated dependence of γb on inclination is confirmed by the measured energies over the entire range. Common to all calculated boundary structures is a microfaceting into {111} and {211} twin facets. The structures ... read less

Abstract: We review recent results on atomic mixing, radiation-induced disordering, and defect production and clustering induced by displacement cascades in Cu, Ag, Cu{sub 3}Au and Ni{sub 3}Al. We employ molecular dynamics computer simulation methods with isotropic many body potentials and recoil energies near subcascade formation regime up the locally molten cascade core in the pure metals and intermetallics. Disordering of intermetallics takes place in the cascade core, but because of the short lifetime of the displacement cascade, chemical short range order is preserved in molten zone. Results reveal very large vacancy and interstitial type defect clusters at high recoil energy and cascade energy density. Vacancies agglomerate and collapse into Frank dislocation loops in quenching of the cascade molten core. Large interstitial clusters are directly produced in cascades and form prismatic dislocation loops. Fraction of defects in clusters for low temperature cascades increases with recoil energy and approaches {approx} 70 and 60% for interstitials and vacancies, at recoil energies near threshold for subcascades. In the case of intermetallics the large energy required to produce and transport a superdislocation appears to inhibit interstitial prismatic loop punching and interstitials appear as isolated (100) dumbbells. read less

Abstract: Detailed Monte Carlo simulations are performed of solute-atom segregation at (002) twist boundaries in the Au–Pt system at 850 K; the particular single-phase bicrystal alloys studied are Pt–1 at% Au and Au–1 at% Pt. The emphasis in this paper is on studying the distribution of solute atoms at low-angle boundaries. For the Pt–1 at% Au alloy the distribution of sites enhanced in the solute species Au is found to form a bipyramid based on the square cells of the orthogonal primary grain boundary screw dislocations. In the case of the Au–1 at% Pt alloy the solute species Pt is found to be depleted and it also forms a similar bipyramidal pattern. The Gibbsian interfacial excesses of Au and Pt are found to be positive and negative, respectively, for the Pt–1 at% Au and Au–1 at% Pt bicrystal alloys. The absolute values of these Gibbsian interfacial excesses both increase with increasing twist angle. An (002)-Dreh-Korngrenzen im Au–Pt System werden ausfuhrliche Monte-Carlo-Simulationen zur Segregation geloster Atome bei 850 K durchgefuhrt; speziell untersuchte einphasige Bikristalle sind Pt–1 At% Au und Au–1 At% Pt. Schwerpunkt der Arbeit ist das Studium der Verteilung der gelosten Atome an Klein-Winkelkorngrenzen. Fur Pt–1 At% Au bilden die Au-angereicherten Platze eine Bipyramide mit den quadratischen Zellen der orthogonalen primaren Korngrenzen-Schraubenversetzungen als Basis. Im Falle der Au–1 At% Pt Legierung findet man auf einem ahnlichen Bipyramiden-Muster eine Abnahme an Pt Atomen. Der Gibbssche Grenzflachen-Uberschus an Au und Pt is positiv bzw. negativ fur Pt–1 At% Au bzw. Au–1 At% Pt Bikristalle. Die Absolutwerte des Gibbsschen Grenzflachen-Uberschusses wachsen mit zunehmendem Drehwinkel jeweils an. read less

Abstract: Quantitative X-ray diffraction measurements have been made from a series of (111) gold bicrystals containing twist grain boundaries with controlled geometry. Each bicrystal was prepared at a specific angle of misorientation Θ° but because of double positioning both Θ° and 60-Θ° twist boundaries were present in the specimens. Since these pairs of boundaries have distinct structures, the intensity measurements from each bicrystal contained superimposed information representing an average from both boundary types. In order to understand these measurements in terms of the boundary structures involved, computer calculations were performed to characterize the theoretical scattering effects expected from [111] twist boundaries related by double positioning. By comparing the measurements with the calculations, it was possible to determine the conditions under which reliable structural information concerning doubly positioned twist boundaries could be obtained. From an appropriate analysis of the data, it... read less